Final answer:
D. Bacteria
Among the options provided, bacteria have the unique ability to become dormant and significantly slow down their metabolism through the formation of endospores, allowing them to survive unfavorable conditions.
Step-by-step explanation:
The type of microorganism that has the ability to slow down its metabolism and become dormant is a bacteria. Bacteria can form endospores, which are resistant structures that allow them to survive in unfavorable conditions. This capability is particularly important for bacteria to acquire resistance to the host's natural defense processes that normally detect and eliminate foreign cells. For example, the bacterium Clostridium tetani, which can live in soil, becomes dangerous when it infects an animal through a wound, particularly in low-oxygen environments where it produces toxins causing tetanus.
Other microorganisms, such as viruses, protozoa, and fungi, have their own ways of surviving adverse conditions or evading the host's immune system. However, the question specifically asks about the ability to become dormant and slow down metabolism, which is a characteristic of bacteria among the options provided.