By induction, to get a string of length 2 , we need 3 productions
S-->PQ , P->k , Q->m
to get a string of length 3 , we need 5 productions
S --> SQ, S --> PQ, P --> k, P --> m
S => SQ => PQQ => kQQ => kmQ => kmm
Thus we need 2n − 1 productions to get string of length n.
Hence option C is correct .