ANS:A - three times

1. Bending Considerations: The main reinforcement in a slab is primarily responsible for resisting bending moments induced by loads. Closer spacing of reinforcement bars helps distribute these bending stresses more effectively throughout the depth of the slab. If the pitch exceeds a certain limit, the slab may experience localized bending stresses that could lead to cracking or failure.
2. Shear Transfer: Proper spacing of reinforcement bars also aids in shear transfer within the slab. Shear forces in slabs can be significant, especially near supports and concentrated loads. Closer spacing of bars ensures that shear forces are adequately distributed and transferred between the bars, reducing the risk of shear failure.
3. Control of Crack Width: Limiting the pitch of reinforcement helps control the width of cracks that may develop in the slab. Narrower spacing of bars restricts the width of cracks that form under loading, which is essential for maintaining durability and serviceability of the structure.
4. Serviceability Criteria: Building codes and design standards often specify maximum limits for reinforcement pitch based on serviceability criteria to ensure that the slab performs satisfactorily under normal usage conditions. Exceeding these limits may result in excessive deflection or cracking, compromising the functionality and aesthetics of the structure.

By limiting the pitch of the main reinforcement to three times the effective depth of the slab, designers aim to achieve a balance between structural integrity, serviceability, and constructability. This ensures that the slab can adequately resist applied loads while maintaining acceptable levels of deformation and cracking.