Acceleration of an object happens when things

slow down

change direction (turn)

go faster

All of the above

all of the above

Explanation:

since acceleration is the change of velocity in respect to time. when things go faster there is a decrease in the time spent and if things slow down there is an increase in time.

since velocity is a vector quantity, so therefore there acceleration happens when there is a change in direction.

both velocity and time point to the answers, so the answer is all of the above.