Which asteroid was the first to be visited by the Galileo spacecraft?

The correct choice is Gaspra, which was the first asteroid visited by the Galileo spacecraft during its mission to Jupiter. This encounter occurred on October 29, 1991, providing a wealth of data about the asteroid's surface, composition, and structure. Gaspra is significant in the study of asteroids because it helped scientists gain insights into the characteristics of such objects in the inner solar system, being among the first to be imaged in detail. The data collected from this encounter contributed to our understanding of asteroid geology and their role in the solar system's history, making Gaspra an important target for early space missions.

In contrast, while Mathilde, Eros, and Kleopatra are also notable asteroids explored by different missions or telescopes, they were not the first to be visited by Galileo. Each of these asteroids came into focus through subsequent missions or studies but did not provide the pioneering observations that Gaspra did during Galileo's journey.