A vector quantity has:

A vector quantity is defined by both its magnitude and its direction. Magnitude refers to the size or amount of the quantity, while direction indicates which way that quantity is acting.

In the context of physics, for example, velocity is a vector quantity. It not only has a speed (magnitude) but also a specified direction (such as "30 km/h north"). Furthermore, the mention of necessary units emphasizes that a complete description of a vector quantity includes the measurement units (like meters per second, newtons, etc.) along with its directional information.

While other answer choices include parts of the correct attributes, they fail to capture the essential combination of direction, magnitude, and the need for appropriate measurement units that fully define a vector quantity. Thus, the correct and comprehensive definition is indeed that a vector quantity has direction, magnitude, and necessary units of the quantity.