**Reinforced Concrete Slab Thickness**

A reinforced concrete slab is a type of structural element that consists of a concrete slab reinforced with steel bars or mesh. The thickness of the slab is an important design consideration as it determines the load-carrying capacity and overall structural performance.

In this question, we are given that the reinforced concrete slab is 128 mm thick. Let's determine the maximum size of reinforcement bar that can be used based on this thickness.

**Reinforcement Bar Size and Spacing Requirements**

Reinforcement bars, also known as rebar, are typically used in concrete construction to provide tensile strength and improve the overall structural performance. The size of the reinforcement bar is specified in terms of its diameter.

In general, the size of the reinforcement bar used in a concrete slab is determined based on several factors, including the thickness of the slab, the required strength, and the spacing requirements. The spacing between the reinforcement bars is also an important consideration to ensure proper distribution of the load.

**Minimum Concrete Cover**

One of the key factors in determining the maximum size of reinforcement bar is the minimum concrete cover required. The concrete cover refers to the distance between the outer surface of the reinforcement bar and the surface of the concrete.

The minimum concrete cover is specified in building codes and standards to protect the reinforcement bars from corrosion and provide fire resistance. It also ensures proper bond between the reinforcement and the surrounding concrete.

**Factors Affecting Maximum Bar Size**

The maximum size of reinforcement bar that can be used in a concrete slab is determined based on the following factors:

1. Concrete cover requirements: Building codes and standards specify the minimum concrete cover required for different types of structures and exposure conditions. The maximum bar size should be selected to ensure that the required concrete cover is maintained.

2. Spacing requirements: The spacing between the reinforcement bars is determined based on the design loads and the strength requirements. The maximum bar size should be selected to ensure that the desired spacing can be achieved.

3. Construction constraints: The size of the reinforcement bars should be compatible with the construction methods and equipment available on-site. It should be feasible to place and compact the concrete around the reinforcement bars.

**Answer: Maximum Bar Size**

Based on the given information that the reinforced concrete slab is 128 mm thick, the correct answer is option 'D' - 16 mm diameter.

The maximum bar size is determined by considering the minimum concrete cover requirements and the spacing requirements. For a 128 mm thick slab, a 16 mm diameter reinforcement bar can be used while maintaining the required concrete cover and achieving the desired spacing between the bars.

Using a larger bar size, such as 20 mm or 25 mm, may not be feasible due to the limited thickness of the slab and the spacing requirements. On the other hand, using a smaller bar size, such as 8 mm or 10 mm, may not provide the desired strength and load-carrying capacity for the slab.

Therefore, option 'D' - 16 mm diameter - is the correct answer based on the given information.