a) 1 part cement, 3 parts fine aggregate and 6 parts coarse aggregate

b) 1 kg cement, 3 kg fine aggregate and 6 kg coarse aggregate

c) 1 part cement, 3 parts aggregate containing both fine and coarse aggregate

d) 1 cubic meter cement, 3 cubic meter fine aggregate and 6 cubic meter coarse aggregate

As per IS 456:2000 and IS 10262:2019, the mix proportions are specified on a volumetric basis unless it is specifically mentioned otherwise. Here, 1 refers to 1 part by volume of cement, 3 refers to 3 parts by volume of fine aggregate and 6 refers to 6 parts by volume of coarse aggregate.

Therefore, option a) is correct. It means that a concrete mix of ratio 1:3:6 contains 1 part cement, 3 parts fine aggregate like sand and 6 parts coarse aggregate like stone chips – all measured by volume. The relative volumes of these ingredients in this mix ratio are 1:3:6.