Select the most appropriate option to fill in the blank and make the sentence meaningful.
You must refrain ________ unpleasant conversations.

Correct option is C

The correct option to fill in the blank is (c) from.
"Refrain" is always followed by "from" + (verb + ing) form. It means to avoid something (से बचना).
Example: You should refrain from arguing in public.
Explanation why others are wrong:
(a) "to" does not fit grammatically.
(b) "with" is used to indicate association, not prevention.
(d) "by" shows method or agent, not prevention.
Information Booster:
After certain verbs like "refrain", "prevent", "prohibit", preposition "from" is mandatory.