Its velocity must be changing.
Explanation
Acceleration is a vector quantity defined as the rate of change of velocity with respect to time. Velocity possesses both magnitude (speed) and direction. Therefore, for an object to undergo acceleration, there must be a change in its velocity vector. This change can occur in the magnitude of velocity, the direction of motion, or both.
Option-wise Analysis
- Its speed must necessarily increase. is Incorrect: Acceleration does not necessarily imply an increase in speed. If the acceleration acts in the direction opposite to the velocity, the speed decreases (retardation or deceleration). In uniform circular motion, the speed remains constant while the direction changes continuously.
- Its velocity must be changing. is Correct: Since acceleration is defined as $\vec{a} = \frac{d\vec{v}}{dt}$, a non-zero acceleration implies that the velocity $\vec{v}$ is changing with time. This is the fundamental condition for acceleration.
- Its mass must be decreasing. is Incorrect: According to Newton's laws of motion, acceleration is the result of a net force acting on a mass ($F=ma$). In classical mechanics, mass is generally treated as constant during motion. A decrease in mass is not a prerequisite for acceleration.
- It must be moving in a circular path. is Incorrect: Acceleration occurs in rectilinear (straight-line) motion when speed changes. It is not limited to circular or curvilinear paths.
Key Takeaway
Acceleration is strictly tied to the change in velocity. An object accelerates if its speed changes, its direction of motion changes, or both change simultaneously.