A vector quantity can only be completely described when ……… and …………are mentioned.

The answer is "Magnitude and direction." A vector quantity is a type of physical quantity that has both magnitude (size) and direction. Magnitude refers to the numerical value of the quantity, while direction refers to the path along which the quantity is moving. For example, if we want to describe the velocity of an object, we need to specify both its speed (magnitude) and the direction in which it is moving. Without knowing the direction, we cannot fully understand the velocity of the object. Similarly, for other vector quantities such as force, acceleration, and displacement, both magnitude and direction must be specified to fully describe the quantity.