The correct answer to the question is option 'B' - shrinkage deflection.

In reinforced concrete structures, the top and bottom reinforcement play a crucial role in providing strength and stability to the members. The reinforcement is placed in such a way that it can resist the tensile forces that develop in the concrete due to various external loads and internal stresses. The main purpose of reinforcement is to control cracks and prevent failure of the concrete.

Unequal top and bottom reinforcement can lead to several issues, including shrinkage deflection. Shrinkage deflection refers to the bending or deformation of a concrete member caused by the shrinkage of the concrete. Shrinkage is the reduction in volume of the concrete as it dries and hardens over time. This shrinkage can cause the concrete to contract, resulting in cracks and deflection.

Here is an explanation of why unequal top and bottom reinforcement can lead to shrinkage deflection:

1. Differential shrinkage: Unequal reinforcement can lead to differential shrinkage in the top and bottom portions of the concrete member. The top reinforcement, which is usually closer to the surface, may be more exposed to drying conditions and can shrink more compared to the bottom reinforcement, which is more protected. This differential shrinkage can cause the member to bend or deform.

2. Tensile stresses: Shrinkage induces tensile stresses in the concrete. These tensile stresses are resisted by the reinforcement. When the top and bottom reinforcement are unequal, the member may experience uneven distribution of tensile stresses. This can result in localized cracking and deflection.

3. Reduced crack control: The purpose of reinforcement is to control cracks in the concrete. When the top and bottom reinforcement are unequal, the crack control capacity of the member may be compromised. The uneven distribution of reinforcement can result in wider cracks and increased deflection.

4. Loss of structural integrity: Unequal top and bottom reinforcement can lead to reduced structural integrity of the member. The differential shrinkage and cracking can weaken the overall strength of the structure and compromise its ability to carry loads effectively. This can result in larger deflections and potential structural failure.

In conclusion, unequal top and bottom reinforcement in a reinforced concrete section can lead to shrinkage deflection. The differential shrinkage, uneven distribution of tensile stresses, reduced crack control, and loss of structural integrity are some of the reasons why this occurs. It is important to ensure proper design and placement of reinforcement to avoid such issues and maintain the stability and durability of the structure.