A: to establish a point from which to measure future distance, or if movement has occurred.
(i do not know the exact definition, so my apologies if this is incorrect, however i do have an explanation)
using process of elimination, we can cross out the 1st, 3rd, and 4th option, as they do not logically make sense:
1 - a reference point is not a system of measures, hence the word ‘point’.
3 - a reference point is again, not a measuring tool or system
4 - at a glance this may make sense, but a reference point is just that, a reference, it’s something you can go back to to see if your work is accurate or has changed in some way, thus option 2 is the best answer.