What is the centripetal force that holds planets in orbit?

If we approximate the orbit of the planets around the Sun to circular orbits with a uniform circular motion, where the velocity is a vector, whose direction is perpendicular to the radius of the trajectory; the acceleration is directed towards the center of the circumference (that's why it's called centripetal acceleration).  

Now, according to Newton's 2nd law, the force is directly proportional and in the same direction as the acceleration:  

Therefore the net force resulting from the movement of a planet orbiting the Sun points towards the center of the circle, this is called Centripetal Force which is a central force that in this case is equal to the gravity force.

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.