Which of the following microorganisms lacks its own metabolic pathways and can only reproduce inside a host cell?

Bacteria are single-celled organisms that possess their own metabolic pathways, allowing them to grow, reproduce, and carry out essential life processes independently. They can thrive in various environments without the need for a host, utilizing their cellular machinery to metabolize nutrients and reproduce through binary fission.

Protozoa are also single-celled eukaryotic organisms that have their own metabolic systems and can reproduce on their own. They are capable of various forms of reproduction, including asexual reproduction through binary fission and sexual reproduction, and can live independently in their environments, such as soil or water.

Muscle is not a microorganism; rather, it is a type of tissue found in multicellular organisms. Muscle cells have their own metabolic pathways and functions, and they do not reproduce independently in the same manner as microorganisms. Therefore, this option does not fit the criteria of the question regarding microorganisms.