Application layer protocol DNS uses TCP and UDP both at the transport layer.
The answer to the question: Which of the following protocols uses both TCP and UDP? is option D, i.e., DNS. The application layer protocol Domain Name Server, abbreviated as DNS, uses both TCP and UDP at the transport layer. It uses port number 53 at TCP for the transfer of information between zones. It uses UDP when a client communicates to the DNS server. Other protocols like FTP and SMTP use only TCP at the transport layer. Telnet uses only UDP at the transport layer.
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