Which equation can be used to solve for acceleration?

the answer is the first one.

Explanation:

one way to find out is if you look at the other equations they are mixed matched. For an example a=d/t should be s= d/t.

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.