Choose the correct option to fill in the given blank :If it__________…

The correct answer is Option (3) → rains

First conditional:

• It is used for a real possibility in the future.
• Pattern: If + simple present, will + base verb.
  • if-clause: simple present (If it rains)
  • main clause: will + base verb (we will cancel)
• Clue word: tomorrow, with "we will cancel", shows a possible future event; the if-clause still takes the simple present, and the singular subject "it" needs rains.
• Complete sentence: If it rains tomorrow, we will cancel the picnic.
  Example: If you hurry, you will catch the bus.

Why the Other Options are Incorrect?

Option 1: rain — Incorrect. Rain is the base form; with the singular subject "it", the simple present takes -s (rains).
Example: I hope it does not rain on Sunday.

Option 2: rained — Incorrect. Rained is the simple past; it belongs to the second conditional (If it rained, we would cancel), not to a sentence with "will".
Example: It rained heavily last night.

Option 4: raining — Incorrect. Raining is the -ing form; it cannot be the verb of the clause without a helping verb (is raining).
Example: Take an umbrella; it is raining outside.

Contrast: a real future possibility takes "If + simple present, will + base verb" (If it rains, we will cancel); an imaginary one takes "If + simple past, would + base verb" (If it rained, we would cancel).