Which best distinguishes between the inner and outer planets?
a. an asteroid belt is the dividing line between the inner and outer planets.
b. the increase in the number of moons divides the inner and outer planets.
c. the inner planets have cooler temperatures than the outer planets.
d. an additional planet found outside the main orbit is the dividing line.

The correct answer is option(A).

Explanation

Inner planets are those planets which are closer to sun, smaller and rockier. For example: Mercury, Earth, Venus and Mars

Outer planets are those planet which far away from the sun with bigger size and are made up of gases.For example: Jupiter, Saturn etc.

There is an asteroids belt present after Mars;s orbit which distinguishes inner planets from outer planets. It is a circumstellar (ring shaped) disc in a solar system composed of small rocks called asteroids.

Hence, the correct answer is option (A).