Why does an acceleration cause velocity to increase

It doesn't always do that.

Acceleration means any change in the speed or direction of motion. So any time an object speeds up, slows down, or turns, acceleration is happening.

These 3 things are all changes of velocity. Acceleration causes them, because that's the definition of acceleration.

answer: the answer is a

real, smaller and upside-down.   so the answer should be b.