The correct answer is A. A vector quantity.
A vector quantity is a quantity that has both magnitude and direction. Velocity is a vector quantity because it tells you how fast something is moving and in what direction it is moving. For example, if you are driving a car at 60 miles per hour north, your velocity is 60 miles per hour north.
A scalar quantity is a quantity that has only magnitude. Examples of scalar quantities include mass, length, and time.
A constant quantity is a quantity that does not change. For example, the speed of light in a vacuum is a constant quantity.
In conclusion, the velocity of a moving body is a vector quantity because it has both magnitude and direction.