Question

how did asteroids form?
continuous collisions of matter create planets
small bits of matter collide together as gravity pulls the matter towards the suns core but it does not collide with larger bodies to become very large
nuclear fusion caused matter to ignite and form asteroids
a comet crashed into the nebula and matter collided to make asteroids

Explanation:

Asteroids are remnants from the early solar system. When the solar system was forming, there was a disk of gas and dust. Small bits of matter (planetesimals) were pulled by gravity (towards the Sun's core region). Some of these small bits collided. But instead of forming large planets (like the major planets we know), they did not collide with large enough bodies to accrete into very large objects. Option 1 is about planet - formation in general (not specifically asteroids). Nuclear fusion is related to star (like the Sun) formation, not asteroid formation. There is no scientific evidence for a comet crashing into a nebula to form asteroids.

Answer:

small bits of matter collide together as gravity pulls the matter towards the Sun's core but it does not collide with larger bodies to become very large