Question

A spaceship moving through deep space will continue moving at the same speed forever. What is the name of the property that allows this to happen? A. Force B. Normal force C. Mass D. Inertia

Answer 1

Inertia allows the spaceship to continue moving through deep space at the same speed forever. 

Answer 2

The best answer is D) Inertia

Inertia is the word assigned to the Newtonian law of motion that an object in motion tends to stay in motion, in a straight line and in the same direction at the same speed, as long as no other forces are acting on it. 
In deep space, there is no air resistance or any other force that would change the ship's velocity, so the law of inertia would keep it moving at the same speed forever.