The correct answer is: D. Diagonal tension.
Shear reinforcement is provided in reinforced concrete (R.C.C.) to resist diagonal tension. Diagonal tension is a type of shear stress that occurs in R.C.C. members when they are subjected to bending. Shear reinforcement is typically provided in the form of steel stirrups or ties.
Vertical shear is the shear stress that occurs in R.C.C. members when they are subjected to vertical loads. Horizontal shear is the shear stress that occurs in R.C.C. members when they are subjected to horizontal loads. Diagonal compression is a type of compression stress that occurs in R.C.C. members when they are subjected to bending.