What position will have the smallest magnitude of vertical velocity?

-- The magnitude of acceleration is the same at all the positions. 
It's 9.8 meters per second-squared downward if this is all happening
on Earth.

-- The smallest magnitude of vertical velocity has to be at the top ... point-B.
In order to change from moving up to moving down, there has to be an instant
when the vertical velocity is zero, and it can't get any smaller than that.