The correct answer is $\boxed{\text{B. }1}$.
The correction factor for momentum is a dimensionless quantity that accounts for the fact that the velocity of a fluid particle is not constant over the cross-section of a pipe. When the velocity distribution is uniform over the cross-section, the correction factor is equal to 1. This means that the average velocity of the fluid particles is equal to the velocity at any point in the cross-section.
The other options are incorrect because they do not account for the fact that the velocity distribution is uniform over the cross-section. Option A, 0, would imply that the average velocity of the fluid particles is zero. Option C, $\frac{4}{3}$, would imply that the average velocity of the fluid particles is $\frac{4}{3}$ times the velocity at any point in the cross-section. Option D, 2, would imply that the average velocity of the fluid particles is twice the velocity at any point in the cross-section.