Correct answer is (b) 28 days
The explanation: Curing is mainly done because the concrete which is laid for the lining gains hardened properties over time. After laying the concrete lining a period of 24 to 36 hours gap is given before curing starts. Curing process is done for a period of 28 days. On the canal bed it is done by constructing 150 mm deep earthen bunds across, so that water stands regularly on the bed. On the side slopes it is done by constructing masonry weep holes.