The speed-time graph for a particle moving at constant speed is a straight-line parallel to the time axis.

Explanation:
When a particle is moving at a constant speed, it means that it covers equal distances in equal intervals of time. This can be represented on a speed-time graph, where the speed of the particle is plotted on the y-axis and the time is plotted on the x-axis.

A straight-line graph indicates that the speed of the particle remains constant throughout the motion. Now, let's analyze each option to determine the correct answer.

a) Perpendicular:
If the speed-time graph is perpendicular to the time axis, it would mean that the speed of the particle is changing at a constant rate. This would result in an acceleration, which is not consistent with the given information that the particle is moving at a constant speed. Therefore, this option is incorrect.

c) Inclined:
If the speed-time graph is inclined, it would mean that the speed of the particle is changing, either increasing or decreasing. This would imply that the particle is accelerating or decelerating, which contradicts the given information that the particle is moving at a constant speed. Hence, this option is also incorrect.

d) Aligned:
This option does not provide any specific information about the relationship between the speed-time graph and the time axis. Therefore, it does not convey any meaningful answer and can be eliminated.

b) Parallel:
The correct answer is option 'B' - parallel. When the speed-time graph for a particle moving at constant speed is parallel to the time axis, it indicates that the speed of the particle remains constant over time. This is consistent with the given information and accurately represents the motion of the particle.

In conclusion, the speed-time graph for a particle moving at constant speed is a straight-line parallel to the time axis.