For which method of determining population size is the population not aware of the sampler’s presence?

The answer is "Indirect observation".

In social research, there are two types of methods used; direct and indirect observation. In this case, since the population is not aware of the research it is clearly indirect observation. In this technique, it observed the effects of results compare to a direct observation which observes as it occurs.

The answer is magnitude measures the effects of an earthquake on the area , while the intensity measures the strength of an earthquake.