Answer:
(B). a forward thrust from the propeller that is equal to the backward force of the water current.
Explanation:
A boat generates thrust when the propellers are turned to accelerate water forward. According to Newton's third law, the resulting thrust pushes the boat in the opposite direction (i.e. backward direction) which is equal in magnitude.
So, the boat moves forward due to a forward thrust from the propeller that is equal to the backward force of the water current.
Hence. option (B) is correct.