ANS:D - increased at the centre of the beam.

1. Shear Force Distribution: In many cases, the shear force distribution in a simply supported beam is highest near the supports and decreases towards the center. However, in certain situations, such as when the beam is subject to concentrated loads or asymmetric loading, the shear force may peak at the center of the beam. Increasing the spacing of stirrups at the center can help accommodate this higher shear force.
2. Flexural Behavior: Beams undergo flexural bending due to applied loads. At the center of the beam, where the bending moment is highest, the beam tends to experience greater shear stresses. As a result, additional shear reinforcement in the form of stirrups may be necessary to resist these higher shear forces effectively.
3. Crack Control: Increasing the spacing of stirrups at the center of the beam can help control the width of cracks that may develop due to shear stresses. By providing closer stirrup spacing where shear forces are expected to be higher, the beam's ability to resist cracking and maintain structural integrity is enhanced.
4. Code Requirements: Some design codes and standards may specify requirements for stirrup spacing based on the expected shear forces along the length of the beam. In certain cases, these requirements may lead to increased stirrup spacing at the center of the beam to meet safety and performance criteria.
5. Structural Optimization: Designers may choose to increase stirrup spacing at the center of the beam as a means of optimizing the structural design while still ensuring adequate shear resistance. This approach aims to achieve a balance between structural efficiency and material economy.

Overall, while it's less common than decreasing stirrup spacing towards the center, increasing stirrup spacing at the center of a beam may be justified under specific loading conditions or design considerations to ensure adequate shear resistance and structural performance.