The correct answer is: C. 0.15%
The minimum reinforcement of the gross sectional area in slabs is 0.15%. This means that for every 100 square centimeters of slab area, there must be at least 1.5 square centimeters of reinforcement. This requirement is intended to ensure that the slab has enough strength to resist bending and cracking.
Option A, 0.10%, is too low. This would not provide enough reinforcement to ensure that the slab has adequate strength.
Option B, 0.12%, is slightly higher than the minimum requirement, but it is still not enough to provide adequate strength.
Option D, 0.18%, is too high. This would result in an unnecessarily heavy and expensive slab.
Therefore, the correct answer is C. 0.15%.