ANS:D - 22 mm

The maximum diameter of a bar used in a ribbed slab typically depends on the design requirements, including factors such as load capacity, span, and spacing of ribs. However, among the options provided, the most commonly used and appropriate maximum diameter for bars in a ribbed slab is: 22 mm This diameter provides adequate strength and structural integrity while also being practical for construction purposes. Larger diameters may be used in specific situations, but they are less common and typically reserved for heavy-duty applications. Therefore, 20 mm is the most suitable choice among the options provided.