The answer is the first one (or A): cells.
Explanation for each option:
A. Cells - Correct. Cells are the simplest body structures considered alive. They are the basic building blocks of all living organisms and are capable of performing all the necessary functions of life, such as metabolism, growth, and reproduction.
B. Tissues - Incorrect. Tissues are groups of similar cells that work together to perform a specific function. While tissues are essential components of living organisms, they are not the simplest structures considered alive.
C. Organs - Incorrect. Organs are made up of multiple tissues that work together to perform specific functions within an organism. Organs are more complex than cells and tissues and are not the simplest structures considered alive.
D. Organ systems - Incorrect. Organ systems are groups of organs that work together to perform complex functions in an organism. They are even more complex than organs and are not the simplest structures considered alive.
Summary:
The simplest body structures considered alive are cells. Cells are the fundamental units of life, capable of performing all the essential functions necessary for an organism's survival.
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