What distinguishes a unicellular organism from multicellular organisms?

A unicellular organism is defined by its composition of a single cell. This characteristic is fundamental to its classification, as it does not have the complexity or the organization of multiple cells that are found in multicellular organisms. In contrast to multicellular organisms, which consist of many cells that can work together to perform various functions, unicellular organisms carry out all life processes within that one cell.

While some of the other characteristics mentioned in the choices can apply to multicellular organisms—in particular, the presence of specialized structures and the presence of a nucleus—these features do not define unicellular organisms. For instance, unicellular organisms can have a nucleus (like protozoa or certain algae), but their defining trait remains that they constitute a single cell, without the division of labor found in multicellular life forms.