The correct option is (a) Maximum.

Shear stress is the internal resistance to the sliding of one layer of material over an adjacent layer due to the applied forces. When a beam is subjected to bending, it experiences both bending and shear stresses. The distribution of shear stress across the cross-section of the beam is not uniform.

To understand why the shear stress in the top fiber of the beam is maximum, let's consider the bending of a simply supported beam. When a beam is subjected to bending, it experiences a moment which causes the top fibers of the beam to be in compression and the bottom fibers to be in tension.

Here's a detailed explanation of why the shear stress in the top fiber is maximum:

1. Shear Stress Distribution: The distribution of shear stress across the cross-section of a beam under bending is triangular. The shear stress is maximum at the neutral axis and decreases linearly towards the top and bottom fibers.

2. Neutral Axis: The neutral axis is the axis along which there is no bending. It passes through the centroid of the cross-section. The top fiber of the beam is farthest away from the neutral axis compared to other fibers, which means it experiences the maximum distance from the neutral axis.

3. Shear Flow: Shear stress is directly proportional to the distance from the neutral axis. As the distance from the neutral axis is maximum for the top fiber, it experiences the highest shear stress.

4. Maximum Bending Moment: The bending moment is maximum at the supports of the beam. This results in higher shear forces at the supports, which in turn leads to higher shear stresses.

5. Combined Effect: The combined effect of the distribution of shear stress and the maximum bending moment results in the top fiber experiencing the highest shear stress compared to other fibers.

In conclusion, when a beam is subjected to bending, the shear stress in the top fiber is maximum. The distribution of shear stress across the cross-section of the beam is triangular, with the highest shear stress occurring at the top fiber due to its maximum distance from the neutral axis and the maximum bending moment at the supports.