Question

the inner planets formed
when the suns heat destroyed all the smaller bodies in the inner solar system.
in the outer solar system and then were deflected inward by interactions with jupiter and saturn.
when a larger planet broke into pieces.
by collisions and mergers of planetesimals.

Explanation:

The current theory of planet formation is the nebular hypothesis. According to this, planets form from the accretion (collisions and mergers) of planetesimals (small solid bodies) in a protoplanetary disk. The inner planets (Mercury, Venus, Earth, Mars) formed in the inner part of the solar nebula. The Sun's heat didn't destroy all smaller bodies (many asteroids still exist). They didn't form in the outer solar system (that's where gas giants formed). There's no evidence of a large planet breaking into pieces to form inner planets.

Answer:

by collisions and mergers of planetesimals.