Which of the following best describes what cells are?

Cells are best described as the smallest, most numerous structural units of the body. They serve as the fundamental building blocks of all living organisms. Each cell carries out essential functions that contribute to the overall health and functioning of the body, such as metabolism, reproduction, and response to stimuli.

In understanding the context of the other options, it’s clear that while groups of cells can form tissues and organs (which are larger structural units), the definition specifically points to cells being the smallest distinct units. Additionally, the largest structural units of the body are typically organs or systems, not cells themselves. While some cells, like lymphocytes, indeed play a role in the immune system, this description does not encompass the full range of cell types and their functions throughout the body. Thus, the statement that cells are the smallest, most numerous structural units captures their essence accurately.