SSC MTS 2 Aug 2019 Shift 3 Paper

Correct option:2) fall - this is the most appropriate option.
Here, 'watched' is the second form of 'watch', it means the sentence is in simple past.
The sentence structure will be as follows :

watched +	him +	fall
↓	↓	↓
Special Verb	Direct Object	Inf. (– to)

Inf. is the base form of the Verb, Hence the correct sentence will be - I watched him fall.