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.

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