Explanation:
The correct answer is option D, DNS (Domain Name System).

DNS (Domain Name System):
DNS is a protocol that is used to convert domain names into IP addresses. It is responsible for translating human-readable domain names into machine-readable IP addresses. DNS uses both TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) depending on the type of query and the size of the response.

TCP (Transmission Control Protocol):
TCP is a connection-oriented protocol that provides reliable, ordered, and error-checked delivery of data between applications. It ensures that all data packets are delivered in the correct order and without errors. TCP is used when the size of the DNS response is too large to fit in a single UDP packet.

UDP (User Datagram Protocol):
UDP is a connectionless protocol that provides fast but unreliable delivery of data. It does not guarantee the delivery of packets or their order. UDP is used when the DNS response is small enough to fit in a single packet and reliability is not as important. It is faster than TCP because it does not have the overhead of establishing and maintaining a connection.

Why DNS uses both TCP and UDP:
DNS primarily uses UDP because it is faster and more efficient for smaller requests and responses. Most DNS queries are small and can fit within a single UDP packet. However, if the DNS response is too large to fit in a single UDP packet, TCP is used instead. TCP allows for larger data transfers and ensures reliable delivery of the DNS response.

Conclusion:
In conclusion, DNS uses both TCP and UDP protocols. UDP is used for smaller requests and responses, while TCP is used for larger responses that cannot fit in a single UDP packet. The use of both protocols allows DNS to efficiently translate domain names into IP addresses while ensuring reliable delivery when needed.