There are four bus routes between A and B; and three bus routes between B and C.

There are 4 bus routes from A to B and 3 bus routes from B to C.

Since, it is used trip so the man with travel back from $C$ to $A$ via $B$.

it is restricted that man cannot use same bus routes from C to B and B to A more than once.

So out of 4 options, 3 are Left from B to A and out of 3 options, 2 are Left from C to B.

∴ for return there are 3 × 2 options and from A to C there are 4 × 3 options.