Select the most appropriate option to fill in the blank.

I watched him ______.

fall

The correct option to fill in the blank is fall.

The sentence is about the speaker watching someone fall. The verb "fall" is the most appropriate word to use in this context because it describes the action of falling.

The other options are not as appropriate.

• To fell is the past tense of the verb "to fell", which means to cut down a tree. It does not make sense to use this verb in the context of the sentence because the speaker is not cutting down a tree, they are watching someone fall.
• To falling is the gerund form of the verb "to fall". It is not grammatically correct to use a gerund form as a verb in this context.
• Fell is the past tense of the verb "to fall". It is grammatically correct to use this verb in the context of the sentence, but it is not as specific as the present tense form "fall".