Excessive deflection in prestressed concrete beams occurs in the post-cracking stage.

Explanation:
- Prestressed concrete beams are designed to withstand high loads and resist cracking. They are constructed by applying compressive forces to the concrete before the loads are applied, which helps to counteract the tensile forces that lead to cracking.
- However, even with prestressing, some cracking may still occur due to various reasons such as shrinkage, temperature changes, or construction errors. These cracks are typically small and do not affect the overall structural integrity of the beam.
- In the pre-cracking stage, the prestressed concrete beam behaves elastically and shows minimal deflection. The prestressing forces are distributed evenly, and the beam remains in its original shape.
- In the plastic stage, the beam starts to deform beyond its elastic limits. The prestressing forces are no longer sufficient to counteract the applied loads, and the beam begins to yield and experience plastic deformation. The deflection increases, but it is still within acceptable limits.
- However, in the post-cracking stage, when cracks have formed in the beam, the deflection becomes excessive. The cracks introduce additional flexibility into the beam, reducing its stiffness and increasing its deflection under load. This excessive deflection can lead to structural problems, such as excessive vibrations, reduced load-carrying capacity, or even failure.
- To mitigate excessive deflection in the post-cracking stage, additional measures are taken, such as providing additional reinforcement or limiting the maximum allowable crack widths.
- Therefore, the correct answer is option 'C' - excessive deflection occurs in the post-cracking stage of prestressed concrete beams.