Final answer:
The correct term for multi-cellular life, as opposed to single-celled protozoa, is Metazoa. Metazoans include a wide variety of multi-cellular organisms such as animals, which are distinct from the single-celled nature of protozoa.
Step-by-step explanation:
If protozoa means single-celled life, then the term used for multi-cellular life is Metazoa. Metazoans are organisms made up of multiple cells, differentiating them from single-celled protozoans. While protozoa are generally nonphotosynthetic, unicellular, and motile, metazoans encompass a broad range of organisms including humans, animals, and insects.
The diversity of protozoa is immense, ranging from organisms that propel themselves with flagella, cilia, or pseudopodia, to those that are stationary. They can be found in both aquatic and terrestrial environments, serving as important constituents of food webs by providing nutrients for other organisms. Despite their single-celled nature, some protozoans, like those of the Apicomplexa group, showcase different cell types although they remain unicellular, which might raise discussions on multicellularity definitions.
Therefore, the correct answer is a) Metazoa, which refers to the broad category of multicellular organisms differentiated from the unicellular protozoa.