If an object moves with constant velocity, then which one of the following statement is not correct?

First, what does "constant velocity" mean? It means the object is moving at a steady speed AND in a steady direction.

Let's look at each option:

Option A: Its motion is along a straight line
If the velocity is constant (steady speed and direction), the object *must* be moving in a straight line. If it curved, the direction would be changing, and thus the velocity wouldn't be constant. So, this statement IS correct.

Option B: Its speed changes with time
If the velocity is *constant*, the speed *cannot* change. The speed is staying the same. So, this statement is NOT correct, and likely our answer.

Option C: Its acceleration is zero
Acceleration is the rate of change of velocity. If the velocity isn't changing (it's constant), then the acceleration is zero. So, this statement IS correct.

Option D: Its displacement increases linearly with time
Displacement is the change in position. If you're moving at a steady speed in a straight line, the distance you cover (displacement) increases at a steady rate. That's what "linearly" means. So, this statement IS correct.

Therefore, the answer is Option B: Its speed changes with time. Because with constant velocity, the speed cannot change.