The problem posed is:
YOU'LL LIKELY SHAKE YOUR HEAD WHEN YOU SEE TELNET AVAILABLE, NORMALLY SEEN ON THIS PORT
Apparently, people gave 21, 22, and 25 as the responses. The correct response, according to RFC assignments of well-known ports, is 23.
But the real correct response is port 21. The problem posed wasn't about which port was assigned to Telnet (port 23), but what you normally see these days.