According to newton’s first law, which characteristic of a moving object would remain constant if there were no other forces acting on it?

The correct answer is the speed.

Explanation:

Newton's First Law, also called the Law of inertia, indicates that "Every body perseveres in its state of rest or uniform rectilinear motion unless it is forced to change its state by forces printed on it."  This then indicates that the speed remains constant, unless a force acts on the body.  

As inertia is the resistance that an object has to a change in its state of motion, that is why Newton's first law is known as the Law of Inertia.

So the speed  would remain constant if there were no other forces acting on it.