What is considered the basic unit of structure and function in all living things?

The correct choice identifies cells as the basic unit of structure and function in all living things. Cells are often referred to as the "building blocks of life" because they are the smallest units capable of performing all necessary life processes, such as growth, reproduction, and response to stimuli. Each cell consists of various components, like the nucleus and organelles, that work together to maintain the life of the organism.

In contrast, tissues are groups of similar cells that come together to perform a specific function, such as muscle or nerve tissue. Organs are structures made up of different tissues working collaboratively to perform complex functions, like the heart or lungs. Systems, on the other hand, are higher organizational levels, comprising multiple organs working together to achieve broader physiological goals, such as the circulatory or respiratory systems. Therefore, while tissues, organs, and systems are essential for the function of organisms, it is the cell that stands out as the foundational unit underlying all biological activity.