A rocket in space can travel without engine power at constant speed in the same direction.?

This is best explained by the concept of...
a) gravitation
b) action-reaction
c) acceleration
d) inertia

Which one and why?

look up newtons three laws. Specifically the law that states...

"An object in motion tends to remain in motion unless acted upon by an outside force"

inertia, because the law of inertia states that an object in motion will stay in motion unless acted upon by an outside force......since there is no friction in space there is no force acting upon the rocket, so the rocket does not need to use fuel, and it can travel at constant velocity

d