The maximum diameter of a bar used in a ribbed slab is 20 mm. This is because the maximum diameter of a bar used in a reinforced concrete structure is limited by the need to ensure that the concrete can adequately bond to the bar. If the bar is too large, the concrete will not be able to bond to it effectively, and the structure will be weakened.
The other options are incorrect because they are either too small or too large. A bar with a diameter of 6 mm would be too small to provide adequate reinforcement, while a bar with a diameter of 22 mm would be too large and would not be able to bond effectively to the concrete.
In conclusion, the maximum diameter of a bar used in a ribbed slab is 20 mm. This is because the maximum diameter of a bar used in a reinforced concrete structure is limited by the need to ensure that the concrete can adequately bond to the bar.