What significant structure does BART use to cross San Francisco Bay?

BART crosses San Francisco Bay using the Bay Bridge, which is a major roadway and transit crossing linking San Francisco and Oakland. The vital role of the Bay Bridge for BART is highlighted by its design and engineering, allowing trains to traverse the bay safely and efficiently. The bridge was specifically designed to accommodate rail transit, making it a crucial component of BART's system.

In terms of other bridges mentioned, while the Golden Gate Bridge is iconic, it is primarily a roadway bridge and does not support BART transit. The Richmond-San Rafael Bridge and the San Mateo-Hayward Bridge are also significant structures in the Bay Area, but they do not serve as transit routes for BART. Thus, the Bay Bridge stands out as the correct answer due to its direct involvement in BART's operation across the bay.