Who was known for her rivalry with the goddess Hera?

Leto is the correct choice as her rivalry with Hera is well-documented in mythology. Leto, the mother of the twin gods Apollo and Artemis, faced significant challenges due to Hera's jealousy and resentment. According to myth, when Leto was pregnant with her children, Hera placed a curse on her to prevent her from finding any place to give birth. As a result, Leto had to search for a remote location to safely deliver Apollo and Artemis, illustrating the lengths to which Hera would go to assert her influence and vendetta.

Hera's animosity towards Leto stemmed from the fact that Zeus, Hera's husband, had been unfaithful to Hera with Leto. This further fueled the conflict between them, deeply embedding Leto's character into the narrative of Hera’s jealousy and rivalry among the gods.

The other options, while significant figures in mythology, do not share the same level of direct conflict with Hera. Demeter is primarily associated with agriculture and motherhood, Athena is the goddess of wisdom and war, and Hecate is connected to magic and the underworld, none of whom were involved in notable rivalries with Hera like Leto was.