What would cause a balloon to expand if taken to the top of a mountain? O A. Increased molecular collision
O B. Increased amount of molecules
O C. Lowered temperature
D. Lowered pressure

D. Lowered pressure

Explanation:

As you go to more altitude or height, the atmospheric pressure significantly lowers so the gas molecules are free to expand and take up as room as possible.

This is best explained by Boyle's law where pressure and volume are inversely related, where if one thing goes up another goes down. Here the pressure goes down, so volume increases and ballon expands.