Fill in the blanks with the most appropriate option:-

The plane was about to ……….. when I boarded.

take off

The correct option to fill in the blank is take off.

The verb "take off" means to leave the ground and begin flying. It is the most appropriate word to use in this context, as the plane was about to leave the ground when the speaker boarded.

The other options are not as appropriate:

• Land means to come down to the ground after flying. It would not be accurate to use "land" in this context, as the plane was not coming down to the ground when the speaker boarded.
• Took off is the past tense of "take off". It would not be accurate to use the past tense in this context, as the plane was not taking off when the speaker boarded.
• Take of is a misspelling of "take off". It would not be accurate to use this misspelling in this context.