Fill in the blank with the correct option. I _____ on Thursday. But o…

The correct answer is Option (2) → was to have left

Was/were to + have + past participle:

• Was/were to + verb describes an arrangement or plan made in the past.
• With have + past participle, it shows that the plan did not happen.
• This sentence: the speaker had planned to leave on Thursday, but a cold stopped him and he waited till Saturday, so "I was to have left on Thursday" fits.
• Complete sentence: I was to have left on Thursday. But on Thursday I had a terrible cold, so I decided to wait till Saturday.

Why the Other Options are Incorrect?

Option 1: am to leave — Incorrect.

• Am/is/are to + verb gives a present arrangement for the future.
• The rest of the sentence is in the past ("had", "decided").
  Example: I am to leave for Delhi tomorrow morning.

Option 3: would leave — Incorrect.

• Would + verb is used for a habit in the past or a future seen from the past.
• It does not show a plan that fell through.
  Example: Every summer we would leave for the hills.

Option 4: will be leaving — Incorrect.

• Will be + -ing is the future continuous, for an action in progress at a future time.
• It clashes with the past "had" and "decided".
  Example: At this time tomorrow, I will be leaving for school.

Contrast: "was to leave" is a plan made in the past. "Was to have left" is a past plan that did not happen.