Final answer:
To describe forces and motions, we need to use components defined in a reference frame and units of size.
Step-by-step explanation:
In the study of motion, the magnitude and directions of forces and motions must be described in terms of an arbitrarily chosen reference frame and arbitrarily chosen units of size.
One of the options given, Components, is correct as it refers to breaking down forces or motions into their separate parts along specified axes.
Magnitude, speed, and acceleration are all important concepts in the study of motion, but they do not specifically refer to the description of forces and motions in terms of reference frames and units of size.