Examples of obligate intracellular parasites that can only replicate inside a host cell are: viruses and prions
rickettsia and viruses
viruses and bacteria
rickettsia and prions

rickettsia and viruses

Explanation:

This is because obligate intracellular parasite are parasite that affect humans and they grow, reproduce and survive inside the host cell. These parasite cannot live or survive outside host cell but they live inside host cell, replicate and they depend on the host cell resources i.e the use host cell resources thereby causing harm on the host cell. rickettsia and viruses are parasite that can only survive inside a living organism.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.