If the velocity-time graph is a line parallel to the time axis, the acceleration of the body is zero.

Here's why:

* Acceleration is the rate of change of velocity. This means how quickly the velocity is increasing or decreasing.

* A horizontal line on a velocity-time graph indicates that the velocity is constant. There is no change in velocity over time.

* If there is no change in velocity, the acceleration is zero.

Think of it this way: If you're driving a car at a constant speed, your speedometer isn't changing. You're not accelerating, even though you're moving.