The correct answer is B. 30 cm.
According to IS: 456, 1978, the thickness of reinforced concrete footing on piles at its edges, is kept less than 30 cm. This is to ensure that the footing is strong enough to support the load of the structure. The thickness of the footing also depends on the type of soil, the load of the structure, and the depth of the piles.
Option A is incorrect because the thickness of the footing should be more than 20 cm to ensure that it is strong enough to support the load of the structure.
Option C is incorrect because the thickness of the footing should be less than 40 cm to ensure that it is not too heavy and does not cause the piles to sink into the ground.
Option D is incorrect because the thickness of the footing should be less than 75 cm to ensure that it is not too heavy and does not cause the piles to sink into the ground.