Answer:
a. The stone thrown upward spends more time in the air.
Explanation:
The stone throw above the horizontal 15° with the same speed as the stone thrown below the horizontal by 15° will remain in the air for a longer time because its vertical component act in the upward direction due to which it first ascends in height and then falls down whereas the stone projected downwards from horizontal will have an additional component of velocity acting in the downward direction due to which it reaches the ground faster.