Question: What is base for the broth in Udon?
About the article "Udon", section "Regional varieties".

Candidate passages:
  "Kake udon" (in Kantō) or "Su udon" (in Kansai): Hot udon in broth topped with thinly sliced green onions, and perhaps a slice of kamaboko.
"Kitsune udon": "Fox udon".
Topped with aburaage (sweetened deep-fried tofu pockets).
"Tempura udon": Topped with tempura, especially prawn, or "kakiage", a type of mixed tempura fritter.
"Tanuki udon" ("Raccoon-dog udon") (in Kantō) or "Haikara udon" (in Kansai): Topped with tempura batter pieces.
"Tsukimi udon": "Moon-viewing udon".
Topped with raw egg, which poaches in the hot soup.
"Wakame udon": Topped with wakame, a dark green sea vegetable.
"Karē udon": "Curry udon".
Udon in a curry-flavoured soup which may also include meat or vegetables.
Biei, Hokkaido is famous for a unique curry udon.
"Chikara udon": "Power udon".
Topped with toasted mochi rice cakes.
"Stamina (sutamina) udon": "Stamina udon".
Udon with various hearty ingredients, usually including meat, a raw egg, and vegetables.
"Nabeyaki udon": A sort of udon hot-pot, with seafood and vegetables cooked in a "nabe", or metal pot.
The most common ingredients are tempura shrimp with mushrooms and an egg cracked on top.
"Kamaage udon": Served in a communal hot-pot with hot water, and accompanied by a hot dipping sauce of dashi and soy sauce.
"Udon-suki": Udon cooked in the manner of sukiyaki.
"Yaki udon": Stir-fried udon in soy-based sauce, prepared in a similar manner to yakisoba.
This originated in Kitakyushu of Fukuoka Prefecture.
(Note that while "yakiudon" is made with udon, "yakisoba" is not made from buckwheat soba, but with steamed Chinese-style ramen.)
"Miso-nikomi udon": hard udon simmered in red miso soup.
The soup generally contains chicken, a floating cracked raw egg that is stirred in by the eater, kamaboko, vegetables and tubers.
The noodles are extremely firm in order to stand up to the prolonged simmering in the soup; additionally, the noodles do not contain salt, so as to avoid over-salting from the salt in the miso.
"Hōtō udon": a local dish of Yamanashi Prefecture, a type of miso soup with udon and vegetables.  
Udon is often served hot as a noodle soup in its simplest form, as "kake udon", in a mildly flavoured broth called "kakejiru," which is made of dashi, soy sauce (shōyu), and mirin.
It is usually topped with thinly chopped scallions.
Other common toppings include tempura, often prawn or "kakiage" (a type of mixed tempura fritter), or "aburaage", a type of deep-fried tofu pockets seasoned with sugar, mirin, and soy sauce.
A thin slice of kamaboko, a halfmoon-shaped fish cake, is often added.
Shichimi can be added to taste.
The flavor of broth and topping vary from region to region.
Usually, dark brown broth, made from dark soy sauce ("koikuchi shōyu"), is used in eastern Japan, and light brown broth, made from light soy sauce ("usukuchi shōyu"), is used in western Japan.
This is even noticeable in packaged instant noodles, which are often sold in two different versions for east and west. 
There are wide variations in both thickness and shape for udon noodles.
 Inaniwa udon (稲庭うどん): a thin type from Akita Prefecture.
 Mimi udon (耳うどん, literally "ear udon"): a lucky preserved food in Kuzu, Tochigi.
It looks similar to ears.
 Himokawa (ひもかわ): an extreme flat and wide type from Kiryū, Gunma.
 Hōtō (rarely 餺飥, commonly ほうとう): a flat and wide type, usually cooked with vegetables, particularly Japanese "kabocha" squash, from Yamanashi Prefecture.
 Kishimen (棊子麺, or more commonly きし麺): a flat type from the Nagoya.
 Ise udon (伊勢うどん): a soft type, usually eaten with sweet soy sauce, from Ise, Mie.
 In Kansai region, a soft and medium thickness type is popular.
 Sanuki udon (讃岐うどん): a thick and rather stiff type from Kagawa Prefecture.
 Hakata udon (博多うどん): a thick and soft type from the Fukuoka.
 Dango-jiru (団子汁): similar to the above "Hohtoh", from Ōita Prefecture.
Nominally a "dumpling soup", it resembles very thick, flat udon.
 Okinawa soba (沖縄そば): also called "suba", a regional Okinawan noodle made by adding some vegetal ash to the flour, similar to how ramen is made.
However, it is very similar to udon.
 There is also a dish called "udon" in Palau, because of the former Japanese administration.
The broth is soy sauce–based like Japanese udon.
However, as there were many immigrants from Okinawa, it uses less broth like Okinawa soba.
Most notably, the noodle is that of spaghetti, as it is easier to acquire there.
Japanese cuisine offers a vast array of regional specialties known as "kyōdo-ryōri" (郷土料理), many of them originating from dishes prepared using traditional recipes with local ingredients.
Foods from the Kanto region taste very strong.
For example the dashi-based broth for serving udon noodles is heavy on dark soy sauce, similar to soba broth.
On the other hand Kansai region foods are lightly seasoned, with clear udon noodles made with light soy sauce. 
Sanuki udon () is a type of udon noodle most popular in the Shikoku region, but is now easily found throughout the neighboring Kansai region and much of Japan.
It is characterized by its square shape and flat edges.
Many udon shops now specialize solely in sanuki udon, and are usually identified by the text "sanuki" in hiragana () appearing in the name of the shop or on the sign.
It is usually served as noodle soup, and was originally popularized in the Kagawa Prefecture of Japan.
The broth is made from a type of dashi which varies in strength and flavor across Japan.
Sanuki udon are named after the previous name of the Kagawa Prefecture, Sanuki Province.