The total physical product is at its maximum when the

The total physical product (TPP) is at its maximum when the marginal physical product (MPP) is zero. This is because MPP represents the additional output gained from adding one more unit of input. When MPP is positive, TPP is increasing. When MPP becomes zero, it means that adding another unit of input does not increase total output, and thus TPP has reached its peak. If MPP were to become negative, TPP would start to decrease.

Therefore, the correct option is A.

The final answer is $\boxed{\text{A}}$