What limits the paramecium from developing more complex functions?

The correct choice highlights a fundamental aspect of cellular biology and organismal complexity. Paramecia, being single-celled organisms, have limitations on how complex they can become due to their size and structure.

As single cells, they must rely on simpler forms of organization to carry out essential life processes. If a paramecium were to develop more complex functions, it would require additional cellular machinery—like organelles and specialized structures—making it larger or more intricate than can be efficiently managed within a single cell. The physical space of the cell is a limiting factor; as cells grow, they encounter challenges with nutrient uptake, waste removal, and maintaining efficient cellular processes.

In contrast, multicellular organisms can develop complex functions because they can dedicate different cells to specialize in various tasks, supporting the organism's overall functioning. This specialization, which is not feasible for a simple unicellular organism like a paramecium, allows for higher levels of organization and complexity in multicellular life's division of labor.

Thus, the space limitation for accommodating more biomachinery directly restricts the potential complexity paramecia can evolve into, confirming why this choice is the most fitting answer.