Final answer:
Bacteria are the dominant life form on Earth when considering cell numbers, and they are classified as prokaryotes, which are unicellular organisms lacking a nucleus. Therefore, the correct option is C.
Step-by-step explanation:
If we use the criteria of cell numbers, then C. Bacteria are the dominant life form on the planet. Bacteria are the most numerous organisms on Earth, and a single bacterium can rapidly multiply into millions under the right conditions. While they can require control for health reasons, bacteria also serve many essential roles, including those necessary for human survival.
Notably, the term prokaryotes is used to describe organisms like bacteria and archaea, which are unicellular organisms without a nucleus. In the broad classification, living things may be single-celled or complex, multicellular organisms such as plants, animals, fungi, bacteria, or archaea. And, according to the first forms of life believed to have appeared on Earth, prokaryotes, specifically bacteria, were among them.