* Its velocity will change. Acceleration is the rate of change of velocity. This change could be an increase in speed (speeding up), a decrease in speed (slowing down), or a change in direction.
Here's why the other options aren't always true:
* It will change direction: While acceleration can cause a change in direction, it doesn't *have* to. An object moving in a straight line can accelerate simply by speeding up or slowing down.
* It will experience a net force: This is true, as Newton's Second Law of Motion states that acceleration is directly proportional to the net force acting on an object (and inversely proportional to its mass). However, the prompt only asks what will *definitely* happen, and other things could cause a net force (like gravity) without causing acceleration.
In summary: The only thing that will *definitely* happen when an object accelerates in a straight line is that its velocity will change.
