Match each application protocol in List-I with its default (well-known) port…

Concept. A port number is a 16-bit transport-layer address that tells a host which application process an arriving segment belongs to. The numbers 0–1023 form the well-known range, which IANA assigns permanently to standard services, so a given service is reached on the same number on every host. Deciding which port belongs to which protocol is therefore recall of a fixed registration, not a calculation.

Applying it here.

1. HTTPS is HTTP carried inside a TLS session. IANA registers it on TCP 443, so K pairs with 443, which List-II labels R.

2. SMTP is the protocol that relays outgoing mail to and between mail servers. IANA registers it on TCP 25, so L pairs with 25, which List-II labels P.

3. DNS resolves names to addresses. IANA registers it on 53 — UDP for ordinary queries, TCP for zone transfers and oversized responses — so M pairs with 53, which List-II labels Q.

Cross-check. The three registrations use each port exactly once, so the mapping is one-to-one: mail takes 25, name resolution takes 53 and secure web traffic takes 443, leaving no protocol without a port and no port claimed twice.

Result. The correct combination is K-R, L-P, M-Q.