D

Step 1: Understand the relationship between acceleration, velocity, and time. Acceleration ($a$) is defined as the change in velocity ($v$) over the change in time ($t$), which can be expressed mathematically as:
$$ a = \frac{\Delta v}{\Delta t} $$
Step 2: When we plot an acceleration-time graph, the area under this graph represents the change in velocity over a given time interval. This is because the area under the curve can be calculated by integrating the acceleration function over time.
Step 3: Therefore, if we take the definite integral of acceleration with respect to time, we can express it as:
$$ \text{Area} = \int a \, dt = \Delta v $$
Conclusion: The area under the acceleration-time graph gives us the change in velocity. Hence, the correct answer is Option D.