* Acceleration is the rate of change of an object's velocity. This means it can be caused by a change in speed, a change in direction, or both.

Let's look at the common ways to accelerate an object:

* Apply a force: Newton's second law of motion states that force equals mass times acceleration (F = ma). Applying a force directly causes acceleration.

* Change the object's mass: While less common, reducing an object's mass while keeping the force constant will increase its acceleration (F = ma).

* Change the object's direction: Even if an object's speed remains constant, changing its direction constitutes acceleration.

Therefore, the answer to your question is:

The option that is not a way to accelerate an object is keeping the object at a constant speed in a straight line.

This is because the object's velocity (both speed and direction) remains unchanged.