MiTM on RSA public key encryption
You need an authentic channel from Alice to Bob to get a secret channel from Bob to Alice. This assumption is missing in a), so anyone in control of the communication channel can play man in the middle on any protocol.

As long you don't have a secret channel from Alice to Bob or an authentic channel from Bob to Alice, Alice will never (= for any protocol) be sure who is sending her the secret messages.

See U. Maurer, P. Schmid, A calculus for secure channel establishment in open networks, ESORICS' 94, LNCS 875, pp 175-192.