Asteroids This is a matter of knowing the definitions. Although the biggest hint is "Make up a belt between Mars and Jupiter" which should cause you to immediately think "Asteroid Belt". But let's look at the options and see what fits.
Comets - Mostly comprised of ice and dust. They're not "rocky", so this isn't a good choice.
Asteroids - Nothing the lecture describes conflicts with what an asteroid is, so this is the correct choice.
Meteor - Key thing here is a meteor is that a meteor is a small body that's burning up in the Earth's atmosphere, causing a streak of light. And that's not what the lecture is describing, so this is a bad choice.
Meteoroids - This is a body orbiting the sun, that will in the future become a meteor. And that's not something in a stable orbit between Mars and Jupiter. So this is also a bad choice.
