Question: Which song did Metallica release in 1991 from their fifth album that is one of their most well known?

Context paragraphs:
Wherever I May Roam: "Wherever I May Roam" is a song by American heavy metal band Metallica.  It was released in October 1992 as the fourth single from their eponymous fifth album, "Metallica".

Sad but True: "Sad but True" is a song by American heavy metal band Metallica.  It was released in February 1993 as the fifth and final single from their eponymous fifth album, "Metallica".  The music video from the single was released in October, 1992.

St. Anger: St. Anger is the eighth studio album by American heavy metal band Metallica, released on June 5, 2003, by Elektra Records.  It was the band's last album released through Elektra, and the second-longest timespan between studio albums from Metallica, with nearly six years between the release of "Reload" and this album.  It is also the final collaboration between Metallica and producer Bob Rock, whose relationship began with the band's fifth studio album, 1991's "Metallica".  "St. Anger" was originally intended for release on June 10, 2003, but was released five days earlier due to concerns over unlicensed distribution through peer-to-peer file sharing networks.  The "St. Anger" sessions also mark the only time Rock played when the music was being recorded, filling in for the departed bassist Jason Newsted.

The Unforgiven (song): "The Unforgiven" is a power ballad by American metal band Metallica.  It was released as the second single from their eponymous fifth album "Metallica" (also known as "The Black Album").  Though one of the slower tracks on the album, its chord progression is distinctly one of the heavier.  The song deals with the theme of the struggle of the individual against the efforts of those who would subjugate him.

AM Radio (song): "AM Radio" is a rock song by the band Everclear.  The song was recorded c. 2000 for Everclear's fourth album "".  The song was released as the second single from Everclear's album "".  It failed to reach the "Billboard" Hot 100, getting to number one on the Bubbling Under Hot 100 Singles chart.  The "AM Radio" single was mistaken by many to be the first single from Everclear's fifth album, "" because the song was released so soon before Everclear's fifth album's release.  The song was later used in a television commercial for General Motors in early 2006.

Metallica (album): Metallica (commonly known as The Black Album) is the fifth studio album by American heavy metal band Metallica.  Released on August 12, 1991 by Elektra Records, it received widespread critical acclaim and became the band's best-selling album.  "Metallica" produced five singles that are considered to be among the band's best-known songs, which include "Enter Sandman", "The Unforgiven", "Nothing Else Matters", "Wherever I May Roam", and "Sad but True". " Don't Tread on Me" was also issued to rock radio shortly after the album's release, but the song did not receive a commercial single release.  The album marked a change in the band's sound from the thrash metal style of the previous four albums to a slower one.  Metallica promoted the album with a series of tours.  In 2003, the album was ranked number 252 on "Rolling Stone"' s 500 greatest albums of all time.

Don't Tread on Me (Metallica song): "Don't Tread on Me" is a song by American heavy metal band Metallica, from the eponymous fifth album, "Metallica".  The title is connected with the American Revolutionary War.  The words "Don't Tread on Me" constitute the motto of the Gadsden flag, and the snake image on the flag is pictured on the cover of the album.

Enter Sandman: "Enter Sandman" is a song by American heavy metal band Metallica.  It was released as the first single from their eponymous fifth album, "Metallica" in 1991.  The music was written by Kirk Hammett, James Hetfield and Lars Ulrich.  Vocalist and rhythm guitarist Hetfield wrote the lyrics, which deal with the concept of a child's nightmares.

Hero of the Day: "Hero of the Day" is a power ballad by American heavy metal band Metallica from their 1996 album "Load".  The song was first recorded on December 13, 1995, and was the first song on "Load" to be recorded.  The song was also Metallica's second single release from the album.  A promotional video for the track was also filmed.  It became their second consecutive number-one hit on the US "Billboard" Hot Mainstream Rock Tracks chart.  It is one of the few Metallica songs written primarily in a major key.

Nothing Else Matters: "Nothing Else Matters" is a song by American heavy metal band Metallica.  It was released in 1992 as the third single from their self-titled fifth studio album, "Metallica".  The song peaked at number 11 on the "Billboard" Mainstream Rock Tracks chart as well as top-ten positions on many European charts.  "Nothing Else Matters" was featured as a playable track in the music video game "".  Recognized as one of Metallica's best known and most popular songs, it has become a staple in live performances.  The song has been covered nearly 100 times.

Supporting paragraphs: Metallica (album) Enter Sandman
Answer: Enter Sandman