Question

I saw the policemen _________ the thieves (chase)

Answer 1

I saw the policeman chasing the thieves

Answer 2

The correct answer to the blank is chasing.

I saw the policemen chasing the thieves.

The verb to be used is chase.

And the sentence is describing the action which which is going on.

So, we should use the verb, verb +ing, that is chase will be chase+ing = chasing.