The correct answer is: C. Parabolic above neutral axis and rectangular below neutral axis.
The shear stress diagram for a reinforced concrete section is parabolic above the neutral axis and rectangular below the neutral axis. This is because the shear stress is proportional to the distance from the neutral axis, and the distance from the neutral axis is greatest above the neutral axis and least below the neutral axis.
The shear stress diagram is used to determine the maximum shear stress in a reinforced concrete section. The maximum shear stress occurs at the neutral axis, and is equal to the shear force divided by the area of the section.
The shear stress diagram is also used to determine the required shear reinforcement. The required shear reinforcement is equal to the maximum shear stress divided by the shear strength of the concrete.
The shear stress diagram is an important tool for the design of reinforced concrete structures. It is used to determine the maximum shear stress in a section, and the required shear reinforcement.