In the zone of R.C.C. beam where shear stress is less than 5 kg/cm2, nominal reinforcement is provided at a pitch of

The correct answer is: A. One-half lever arm of the section.

The lever arm is the distance from the neutral axis to the outermost fiber of the beam. The nominal reinforcement is provided at a pitch of one-half the lever arm of the section to ensure that the shear stress is less than 5 kg/cm2.

The other options are incorrect because they do not provide the required shear reinforcement.

• Option B: One-third lever arm of the section. This option would not provide enough shear reinforcement and the shear stress would exceed 5 kg/cm2.
• Option C: Lever arm of the section. This option would provide too much shear reinforcement and the beam would be unnecessarily heavy.
• Option D: One and half lever arm of the section. This option would provide too much shear reinforcement and the beam would be unnecessarily heavy.

Therefore, the correct answer is A. One-half lever arm of the section.