The method in which the population is not aware of the sampler's presence is indirect observation (Option d).
Indirect observation refers to the collection of information obtained by other researchers in order to record it in specific informative materials (e.g., books, review/research papers, videos, etc.).
An example of indirect observation can be the record in a book of specific animal behaviors (e.g., the presence of marks on the ground).
In conclusion, the method in which the population is not aware of the sampler's presence is indirect observation (Option d).
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