Relative clause

Who says there's something wrong with it?

This is the TOEFL test. The answer key went for option 2.It is gravity ---------- objects toward the earth.1) pulling 2) that pulls 3) to pull 4) what pulls

2) is the correct ansawer.

Freeguy It is gravity pulling objects toward the earth.What's wrong with "pulling" here?

"that pulls" is correct.

FreeguyWhat's wrong with "pulling" here?

Nothing, if it refers to something previously mentioned. A present participle (pulling) is often used in active relative clause equivalents.
Cf. Look at that! It is a cat chasing a mouse.

CB

If "pulling" works, why don't you choose #1 here?

Freeguy It is gravity pulling objects toward the earth. What's wrong with "pulling" here?

Nothing wrong at all, if you want to give a sort of 'progressive' (dynamic) sense to the meaning.

As a timeless statement of fact (or 'eternal truth'), which it is, then no. 2 (the relative construction) with its present tense "It is gravity that pulls objects to

Freeguy This is the TOEFL test. The answer key went for option 2.It is gravity ---------- objects toward the earth.1) pulling 2) that pulls 3) to pull 4) what pulls

What do the instructions say?