D

To analyze the motion of a particle under constant negative acceleration starting with positive velocity, we can consider the implications of the velocity-time graph.

Step 1: Understanding the Scenarios
1. The particle is initially moving in the positive direction, which implies a positive initial velocity (v > 0).
2. Constant negative acceleration means that the velocity will decrease over time, but it will remain positive until it reaches zero.

Step 2: Examining the Graphs
- Option (W): The velocity rapidly decreases to zero, which is consistent with a particle coming to a stop smoothly.
- Option (X): The velocity graph indicates that the particle accelerates in the positive direction, which contradicts our scenario.
- Option (Y): The velocity is increasing, indicating the particle is moving faster over time, which is also not in line with the negative acceleration.
- Option (Z): This graph shows the velocity decreasing over time until it becomes zero and then continues into negative velocity, indicating that the particle comes to a stop and then moves in the opposite direction.

Conclusion
The graph that represents a particle initially moving in the positive direction with constant negative acceleration is option (Z). After a certain time, the velocity decreases to zero and continues as a negative velocity, demonstrating a smooth transition from positive to negative velocity.