To determine the solution, we utilize the relationship between acceleration and velocity. The acceleration-time graph depicts acceleration's change over time, and the velocity-time graph is derived by integrating acceleration with respect to time.
Step 1: Understanding the Acceleration-Time Graph
From the given acceleration-time graph:
1. Acceleration is constant for the first interval (from \( t = 0 \) to \( t = 6 \)).
2. Acceleration is also constant for the second interval (from \( t = 6 \) to a higher value).
Step 2: Velocity-Time Graph from Acceleration-Time Graph
Velocity is the integral of acceleration over time. Since acceleration is constant in each interval, the velocity-time graph will show a linear increase during intervals with non-zero acceleration.
- In the first interval (constant acceleration), velocity increases linearly.
- In the second interval (constant acceleration), velocity continues to increase linearly, with a potentially different rate based on the acceleration value.
Step 3: Analyzing the Options
- Option (A): A trapezium-shaped graph implies a non-linear increase, which is incorrect due to constant acceleration.
- Option (B): A right-angle triangle-shaped graph is also incorrect as it suggests a sharp, right-angled slope.
- Option (C): An L-shape suggests abrupt velocity changes, inconsistent with constant acceleration.
- Option (D): This option shows a linear velocity increase, consistent with constant acceleration.
Step 4: Conclusion
The correct velocity-time graph displays a linear velocity increase over time due to constant acceleration.
Thus, the correct answer is:
\[
\boxed{(D)}
\]