The correct answer is: C. 10 mm
The maximum permissible size of aggregates to be used in casting the ribs of a slab is 10 mm. This is because the aggregates should be small enough to fit through the reinforcement bars in the slab, but large enough to provide good concrete strength. If the aggregates are too small, they will not provide enough strength. If the aggregates are too large, they will not fit through the reinforcement bars and will create voids in the concrete.
Option A: 5 mm is too small for the aggregates to provide good concrete strength.
Option B: 7.5 mm is a good size for the aggregates, but it is not the maximum permissible size.
Option C: 10 mm is the maximum permissible size for the aggregates.
Option D: 15 mm is too large for the aggregates to fit through the reinforcement bars.