According to I.S.: 456, 1978 the thickness of reinforced concrete footing on piles at its edges, is kept less than A. 5 cm B. 10 cm C. 15 cm D. 20 cm

The correct answer is: B. 10 cm.

According to I.S.: 456, 1978, the thickness of reinforced concrete footing on piles at its edges, is kept less than 10 cm. This is to ensure that the footing is strong enough to support the load of the structure, but not so thick that it becomes unnecessarily expensive.

Option A: 5 cm is too thin and may not be strong enough to support the load of the structure.

Option C: 15 cm is a common thickness for reinforced concrete footings, but it is not necessary for footings on piles.

Option D: 20 cm is too thick and may be unnecessarily expensive.