Final answer:
A microorganism in a resting state with a hard, shell-like outer covering is known as a spore.
Step-by-step explanation:
A microorganism that is resting and has a hard shell-like outer covering is called a spore. This definition fits into the broader context of protozoan life cycles, where some protozoa can enter a protective state called a cyst, which is similar to a spore in that it has a protective wall. For instance, the genus Eimeria, which includes certain pathogens, is capable of such a process known as encystment, where the protozoan becomes a cyst to withstand harsh environmental conditions. The protozoa then return to an active state known as excystment when conditions are more favorable. However, the direct answer to the multiple-choice question provided is spore, as protozoa, bacteriophages, and algae do not inherently refer to a resting organism with a hard, shell-like outer covering.