If an object moves with constant velocity, it means that its speed and direction remain constant over time. Let`s analyze each option to understand why statement 2 is NOT correct.

Option 1 states that the object`s motion is along a straight line. This is correct because constant velocity implies that the object is moving in a straight line without deviating from its path.

Option 2, which is the correct answer, states that its speed changes with time. This statement is NOT correct. When an object is moving with constant velocity, its speed remains constant. It does not increase or decrease over time.

Option 3 states that its acceleration is zero. This is correct because constant velocity implies that there is no change in velocity or acceleration. The object is neither speeding up nor slowing down.

Option 4 states that its displacement increases linearly with time. This statement is correct. When an object moves with constant velocity, its displacement increases linearly with time. This means that as time passes, the object covers an equal distance in equal intervals of time.

In conclusion, statement 2 is not correct because when an object moves with constant velocity, its speed does not change with time.