ANS:D - diagonal tension.

The shear reinforcement in reinforced concrete (R.C.C.) is provided to resist diagonal tension. Here's why:

1. Diagonal Tension: When a beam or slab is subjected to shear forces, it can develop diagonal tension cracks near the supports or concentrated loads. These cracks form diagonally from the point of application of the shear force towards the supports. Diagonal tension is the primary failure mode in concrete elements subjected to shear.
2. Shear Reinforcement Purpose: Shear reinforcement, often in the form of stirrups or inclined bars, is provided in reinforced concrete elements to resist and control these diagonal tension forces. The stirrups help to close and prevent the widening of diagonal tension cracks, thereby enhancing the shear capacity of the member.
3. Vertical Shear: While shear reinforcement indirectly assists in resisting vertical shear by improving the overall shear capacity of the member, its primary function is to address diagonal tension specifically.
4. Horizontal Shear: Horizontal shear is not a typical failure mode in reinforced concrete members under normal loading conditions. Therefore, shear reinforcement is not specifically provided to resist horizontal shear.
5. Diagonal Compression: Diagonal compression occurs in concrete elements under compression, especially near the supports. However, shear reinforcement is not provided to resist diagonal compression. Instead, it's intended to address diagonal tension, which is the opposite effect.