Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Problem: a bookseller has two display windows. she plans to display 3 new fiction books in the left window, and 3 new non - fiction books in the right window. assuming she can put the 4 fiction books in any order, and separately, the 3 non - fiction books in any order, how many total configurations will there be for the two display windows ?
Options: a. 24, b. 72, c. 144, d. 36, e. 420
d