Choose the correct option to fill in the blank:If I were you I ______…

The correct answer is Option (4) → would refuse

Second conditional (an imaginary situation):

• Form: If + simple past (were) ..., subject + would + base verb.
• It describes an unreal present situation and its imagined result; "If I were you" is the usual way of giving advice by imagining oneself in another person's place.
• This sentence: Clue words: "If I were you". The condition is unreal (the speaker is not the listener), so the result clause takes would refuse.
• Complete sentence: If I were you I would refuse to accept the bribe.
  Example: If I were rich, I would travel round the world.

Why the Other Options are Incorrect?

Option 1: should refuse — Incorrect. Should here would express duty or advice (ought to); in modern English the result clause of the second conditional takes would ("I should" for "I would" is an older British use).
Example: You should see a doctor about that cough.

Option 2: can refuse — Incorrect. Can expresses present ability or possibility and belongs to real conditions; it does not follow "If I were you".
Example: You can borrow my bicycle if you like.

Option 3: must refuse — Incorrect. Must expresses strong obligation; it is not used for the imagined result of an unreal condition.
Example: All visitors must sign the register.

Contrast: an unreal condition ("If I were you") takes would + base verb in the result clause; "can" belongs to real conditions, and "must" and "should" express obligation or advice.