What is the centripetal force that holds planets in orbit?

a. inertia
b. gravitational force
c. planetary force
d. kepler's force

The force is B. gravitational force.

A force from a fixed center point which directs the moving body towards it is called centripetal force. This force makes the body to follow a curved path.

In the case of sun and earth, the force which makes the earth follow a particular orbit and hold it in the same orbit is the gravitational force exhibited by the sun.  

Gravitational force is a pulling force that attracts objects with mass. Every object in the universe is pulling on another object even when they are very far from each other.

you cant move and you feel like someones pushing you down onto your bed it can feel like you want to get up but you just can't it can be extremely scary

show the wavelengths of a wave. show the crests of a wave.