a) SMTP - It is a application layer protocol for sending email.

b) IP - It is a network layer protocol.

c) TCP - It is a connection oriented transport layer protocol.

d) UDP - It is a connectionless transport layer protocol.

SMTP is an application layer protocol used for sending e-mail, it is a connection oriented protocol, therefore it uses TCP at transport layer.

Therefore, SMTP in application layer uses TCP in transport layer for mail services.