By definition we have to:
A system or frame of reference are those conventions used by an observer (usually standing at a point on the ground) to be able to measure the position and other physical magnitudes such as speed and acceleration of one or several objects.
The numerical value of some magnitudes can also be relative to the reference system when we refer to the relative movement. There are always mathematical relationships between the observer and the relative magnitudes.
Answer:
C. a position from which something is observed