I used 'port' without an article because when on a ship, one does not use an article (one steers to port or to starboard - it's a direction, not a particular port on the shore).
I'm confused because I assumed that the speaker is stating the rule: when looking toward the bow of a ship, port is on the left side and starboard is on the right. So I put in, 'Port is on the left side' and got, 'You need an article' and the answer 'A port is on the left side.' I came here to see if others had the same confusion and see that the answer given here is 'THE port is on the left side.'
'Port is on the left side' - means 'port is on the left side of a ship' (and starboard is on the right side of a ship when facing toward the bow.) This is a well-known rule, even to non-sailors.
'A port is on the left side' - Some unidentified port is on the left side of something, not necessarily a ship. (A port is on the left side of the fish cannery,'' for example.)
'The port is on the left side' - some port, known to speaker and listener, is on the left side of some thing, also known to the speaker and listener.
I submit that all three of those answers should be counted as correct.