Answer :
(D) indirect observation is the right response since it is a method of determining population size is the population not aware of the sampler's presence.
The population is not aware of the sampler's presence when indirect observation is used to estimate population size. The population's members may be few or elusive when employing indirect observation.
It can be simpler to look for their footprints or other traces than the actual organisms. As animals are viewed from a distance without being in contact with them, this technique is the least invasive.
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