Astronomers beleive that there is no planet in the asteroid belt region because
a. collisions in the asteroid belt break apart objects instead of letting these objects grow through accretion.
b. the magnetic field of jupiter prevented the material in the asteroid belt from gathering into a planet.
c. the asteroid belt is highly inclined to the orbits of the planets so there was not enough material to form a planet.
d. orbital resonances with jupiter disrupted the orbits of polanetesimals in this region.

Option d.

Explanation:

The asteroid belt is a stellar disc which exists in between Mars and Jupiter's orbits which consists of a large no. of asteroids in our solar system.

the asteroids in the asteroid belt are formed from different regions and those asteroids with the similar chemical properties or compositions form a group and the different groups formed on these basis are distant from each other with some gap called as kirkwood gap. The gaps so formed are in resonance with Jupiter and are influenced by the strong gravitational force of Jupiter which renders them unstable in the region of resonance which prevents planet formation.