Answer:
Option A is correct
Step-by-step explanation:
As the piano moves upwards, the net force acting on the piano is
Fnet = T - mg
Where T be the tension in the string and mg be the weight of piano.
Here Fnet is always positive because piano is moving up and upward direction is taken as positive.
When the piano stops, the net force acting on the piano is zero.
Fnet = 0
When the piano moves downwards, the net force acting on the piano is
Fnet = mg - T
As the net force is acting downwards, so the direction of force is negative.