Assertion (A) : When a body is dropped from an aeroplane moving horizontally with constant velocity, its trajectory is a vertical straight line for the pilot of the plane.
Reason (R) : Aeroplane and the body dropped from it move with same velocity along horizontal direction.

From the instant the body is dropped from the aeroplane the body will also have a horizontal velocity, so the relative velocity initially will be zero and this will remain the same in horizontal direction.

But the body will have a velocity of 'g' downwards w.r.t the aeroplane.