* Scalar Quantity: A scalar quantity has only magnitude (size or amount). Speed is a scalar quantity because it tells you how fast something is moving, but not in what direction.

Let's break down the other options:

* A. A quantity with direction only: This describes a vector quantity, not a scalar quantity.

* B. no units: Speed has units, like miles per hour (mph), kilometers per hour (km/h), or meters per second (m/s).

* C. vector: A vector quantity has both magnitude and direction. Velocity, which is speed with a direction, is a vector.

In summary: Speed is a scalar quantity because it only describes how fast something is moving, not in which direction.