What defines a scalar quantity?

A scalar quantity is defined as a quantity that possesses only size or magnitude, without any directional component. This is why the choice indicating that a scalar has only size is the correct answer. Scalars are characterized by their numerical value and units, such as temperature, mass, speed, and energy, allowing them to be fully described by a single number.

In contrast, other types of quantities involve direction, such as vectors, which include both magnitude and direction, such as velocity and force. Therefore, the other options that mention direction or motion do not accurately describe scalar quantities, which are strictly represented by their size without needing to reference any direction in space.