What can be defined as the basic building blocks of all body tissues?

Cells are indeed defined as the basic building blocks of all body tissues. They are the smallest functional units of life and are responsible for carrying out the necessary processes to sustain an organism. Each cell can perform various functions that contribute to the overall operation of tissues, which are groups of similar cells working together to perform specific tasks.

Tissues, the second choice, are composed of groups of cells that share a common structure and function. While they are essential components of the body's organization, they cannot be considered the basic building blocks themselves, as they are made up of cells.

Organ systems refer to groups of organs that work together to perform complex functions, and they are built from tissues, which in turn are made from cells. Thus, they are higher levels of organization that rely on the foundational role of cells.

Atoms, while essential as the basic units of matter that make up everything in the universe, including cells, are not specific to biological structures and functions. Cells represent the first level of the hierarchy in living organisms, making them the fundamental building blocks of body tissues.