What is the primary composition of the Monera kingdom?

The primary composition of the Monera kingdom is mostly unicellular prokaryotic organisms. This kingdom is specifically designed to encompass organisms that lack a defined nucleus and membrane-bound organelles, characteristics that define prokaryotes. Organisms within Monera include bacteria and archaea, which play vital roles in various ecological processes, such as decomposition and nitrogen fixation.

In contrast, multicellular organisms with complex structures belong to different kingdoms, such as Animalia or Plantae, indicating a level of complexity not found in Monera. While Monera includes microorganisms, the statement that it consists of "only microorganisms" is too limiting, as it also includes significant diversity within prokaryotic life, such as extremophiles. Similarly, complex plants are classified under a different kingdom (Plantae) and therefore are not part of Monera. Understanding these distinctions clarifies why the correct answer emphasizes the unicellular and prokaryotic nature of the organisms in this kingdom.