B

Step 1: Calculate the total work done. The work done (W) to lift one stone to a height (h) is given by the formula:
W = mgh
Where:
m = mass of one stone = 1 kg
g = acceleration due to gravity = 10 m/s²
h = height = 6 m
Thus, the work done for one stone:
W = 1 kg × 10 m/s² × 6 m = 60 Joules.
Step 2: Since there are 100 stones, the total work done (W_total) is:
W_total = 100 stones × 60 Joules/stone = 6000 Joules.
Step 3: Calculate the time taken in seconds. Since the labourer took 2 minutes, we convert this to seconds:
Time = 2 minutes × 60 seconds/minute = 120 seconds.
Step 4: Calculate the average power (P) using the formula:
P = \frac{W_{total}}{t}
Thus, P = \frac{6000 Joules}{120 seconds} = 50 Watts.
Therefore, the average power is 50 Watts.