Answer:
a. relative size
Explanation:
Relative size -
It refers to as the ability , that allows to find how far or close is the object , according to the size , is referred to as relative size .
This is a pure estimation , and hence can be false in some or the other way .
It is not the exact or genuine method , but is quite effective .
Human being uses this concept to find the size of moon , on comparing with some other heavy body , as the exact measurement of the size is near to impossible .
Hence , from the given scenario of the question,
The correct option is a. relative size .