Distribution of shear intensity over a rectangular section of a beam, follows:

Shear stress distribution in a rectangular cross-section of a beam is not uniform across the depth.

The intensity of shear stress is zero at the extreme top and bottom fibers and maximum at the neutral axis (center of the depth).

This distribution follows a parabolic curve, where the shape of the curve resembles a parabola with the vertex at the neutral axis.

This is derived from the shear stress formula in bending: τ = VQ / Ib, where τ is shear stress, V is shear force, Q is the first moment of area, I is the moment of inertia, and b is the width.

Hence, for rectangular sections, the resulting variation of τ across the depth is parabolic in nature.

Therefore, the correct answer is Option C: A parabolic curve.