Option:- C is right answer S3, S1, S2, S4
As Bob want to send email to Alice :-
1. (S3) Very first at Bob end, SMTP with help of TCP is used to transfer email from Bob outlook client to Bob's outlook server.
2. (S1) DNS with help of UDP at Bob's outlook server will get Alice mail server IP address.
3. (S2) SMTP with help of TCP is used to transfer e-mail from Bob's outlook server to Alice's Gmail Server.
4. (S4) HTTP with help of TCP is used to read e-mail from Alice's Gmail server to Alice Gmail client in Web Browser.
If you think logically,
1. When bob write email and click send, the email is not on the way to Alice. Because according to email architecture, the send email need to visit first it's own email server. The email client knows the IP address of it's own email server. so it simply make use of SMTP protocol to establish TCP connection and transfer it reliably to own server(Bob's server). In this step UDP is not used.
2. a. Now Bob's email server will make use of UDP to get the destination(Alice mail server) IP address.
b. After getting IP address, with the help of SMTP protocol transfer the email packet from Bob's email server to Alice email server reliably.
3. Now Alice email server receives email packet. This email packet need to be send to Alice email client whenever Alice request it . Alice email server know Alice email client IP address. So Alice client simply make use of HTTP / IMPA4 / POP3 protocol to read the email from it own email server.