Correct Answer - Option 3 : 15 cm
According to I.S. 456-1978, the thickness of reinforced concrete footing on piles at its edges is kept less than 15 cm.
However, from the updated IS Module of 456:2000:
As per IS 456: 2000, Clause 34.1.2,
Thickness at the Edge of Footing
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils.
For footings on piles, the thickness at the edge shall be not less than 300 mm (30 cm) above the tops of piles.