Which earlier Akkadian epic shares the flood story with Gilgamesh?

The earlier Akkadian epic that shares the flood story with Gilgamesh is Atrahasis.

Atrahasis is an Akkadian poem preserved in cuneiform tablets, with a major version dating to the 18th century BCE. It tells how the gods create humans, become troubled by humanity’s noise and growth, and send a devastating flood. The wise man Atrahasis survives after the god Enki secretly warns him to build a boat.

The flood episode in Tablet XI of the Standard Babylonian Epic of Gilgamesh closely follows the Atrahasis tradition. In Gilgamesh, however, the survivor is called Utnapishtim rather than Atrahasis, and the story is embedded in Gilgamesh’s quest for immortality. Scholars see the Gilgamesh version as adapting or reworking earlier flood material, although the precise transmission history remains uncertain.

The other options belong to different Mesopotamian works: Descent of Ishtar concerns the underworld, Lugal-e is a hymn about Ninurta, and Enuma Elish is a creation epic. Atrahasis is therefore the single best answer.