Final answer:
The significant geological event that happened in the Long Valley-Mammoth Lakes area of California about 760,000 years ago was caldera formation (B).
Step-by-step explanation:
The significant geological event that happened in the Long Valley-Mammoth Lakes area of California about 760,000 years ago was Caldera formation (B).
A caldera is a large volcanic crater that forms when a volcano collapses or explosively erupts. In this case, the Long Valley Caldera was created by a massive volcanic eruption, leading to the formation of a large depression in the Earth's surface. The eruption was so powerful that it released a large amount of volcanic ash and debris, which covered the surrounding area.