What distinguishes the cells of unicellular organisms from those of multicellular organisms?

The distinction highlighted in the correct answer focuses on the specialization of cells within multicellular organisms compared to unicellular organisms. In multicellular organisms, cells differentiate into various types that perform specific functions, such as nerve cells for signaling, muscle cells for movement, and epithelial cells for protection. This specialization allows for more complex biological systems and functions that enable the organism to survive and thrive. Each type of cell contributes to the overall function of the organism, collaborating with other specialized cells.

In contrast, unicellular organisms operate as single cells that must perform all necessary life functions independently. This includes processes such as metabolism, reproduction, and response to the environment, all carried out within one cell. While some unicellular organisms can exhibit a variety of life processes and adapt to their environments, their lack of cellular specialization limits them compared to multicellular systems.

The other choices do not accurately capture the main distinction as illustrated by the specialization of multicellular organisms. The size of cells or the simplicity of unicellular organisms being prokaryotic does not universally apply, as multicellular organisms can have small or large cells and unicellular organisms can also be eukaryotic.