Acceleration and its S.I. unit
Acceleration is the rate of change of velocity with respect to time. It is a vector quantity, meaning it has both magnitude and direction. The standard unit of acceleration in the International System of Units (S.I.) is meters per second squared (m/s^2).

S.I. unit of acceleration
The correct S.I. unit of acceleration is ms^-2. This unit represents meters per second squared, indicating that the velocity of an object changes by 1 meter per second for each second that passes.

Explanation
- Acceleration is calculated by dividing the change in velocity by the time taken for that change to occur. Since velocity is measured in meters per second (m/s) and time is measured in seconds (s), the unit of acceleration becomes meters per second squared (m/s^2).
- The negative exponent (-2) in the unit indicates that acceleration is a change in velocity per unit time, squared. This means that acceleration is not only the change in velocity per second but also how that change accumulates over time.
- It is important to note that the unit of acceleration is a combination of units of length (meters) and time (seconds), highlighting the fundamental relationship between acceleration, velocity, and time.
In conclusion, the S.I. unit of acceleration is ms^-2, representing the change in velocity per unit time squared. Understanding this unit is crucial in physics for describing the motion of objects and analyzing their behavior.