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.