- When shall we hold the meeting?
- It ____ be better to put it off until next week.
A. mustTo me, would works well because (i) the phrase it would be better is a fixed phrase, (ii) its conditional sense, and (iii) the function of the structural subject It:
Conditional ItThe key (B. could) is probably a typo and should read B. would.
would be better if
we put it off ... Existential-it
It would be better to put it off until next week
. Putting it off until next week would be better
It will be better when you see a dentist.
Your tooth/Life will be better when you see a dentist.