Diagonal Crack due to Tension:
In case of deep beams or thin webbed RCC members, the first crack that forms is a diagonal crack due to tension. This crack occurs due to the tensile forces acting on the member.

Explanation:
When a deep beam or a thin webbed RCC member is subjected to loads, the upper part of the beam experiences compression while the lower part experiences tension. As a result, the beam undergoes bending and shear stresses.

Flexural Crack:
Flexural cracks occur in beams due to bending stresses. These cracks are vertical and form at the bottom of the beam where tension forces are maximum. However, in the case of deep beams or thin webbed members, the web depth is relatively smaller compared to the overall depth of the beam. This leads to the formation of diagonal cracks due to tension instead of flexural cracks.

Shear Crack:
Shear cracks occur in members subjected to shear forces. These cracks are inclined and form at an angle of approximately 45 degrees to the horizontal. However, in the case of deep beams or thin webbed members, shear cracks are not the first cracks to form. Diagonal cracks due to tension develop before shear cracks.

Diagonal Crack due to Compression:
Compression cracks occur in members subjected to compression forces. These cracks are vertical and form at the top of the member where compression forces are maximum. However, in the case of deep beams or thin webbed members, diagonal cracks due to compression do not form as the compression forces are distributed over a larger area.

Therefore, the correct answer is option 'C' - diagonal crack due to tension. The tension forces acting on the lower part of the deep beam or thin webbed member cause diagonal cracks to form. These cracks help in redistributing the stresses and improving the load-carrying capacity of the member.