Right choice is (a) True
Easy explanation: Flatter slopes such as 1 : 1 and 1.5 : 1 are also constructed at the time of execution, but actual capacity of channel is worked out in 1/2 : 1 slope because even if we provide 1 : 1 or 1.5 : 1 slope the silt gets deposited on this slope and gives a new side slope 1/2 : 1. Therefore the actual capacity is calculated using 1/2 : 1 slope.