Fill in the blank with the suitable Modal. If I were a bird I _______…

The correct answer is Option (4) → would

Note: The given answer is as per the official NTA Answer Key. However, more than one answer is possible here: Option (3) "could" and Option (4) "would" both make a correct sentence, as explained below.

Option 1: should — Incorrect. In present-day English, should expresses duty or advice; "If I were a bird I should fly" reads as an obligation, not as the imagined result. (Its use as a conditional with "I" is old-fashioned British usage.)
Example: You should drink more water in summer.

Option 2: must — Incorrect. Must expresses strong obligation or necessity; it is not used for the result of an unreal condition.
Example: Students must carry their identity cards.

Option 3: could — Can also be considered as correct.

• Could expresses ability or possibility in an imagined situation.
• "If I were a bird I could fly in the sky" is a standard sentence: as a bird, the speaker would have the ability to fly.
  Example: If I had a car, I could drive you home.

Option 4: would — Correct (Answer as per NTA).

• Unreal (second) conditional: it imagines a situation that is not true now.
  • If-clause: If + subject + past tense, with were for every person: "If I were a bird";
  • main clause: subject + would + base verb: "I would fly in the sky".
• "Would" gives the imagined result of being a bird.
  Example: If I were rich, I would travel round the world.

Complete sentence: If I were a bird I would fly in the sky.