Blood consists of many different types of cells. These cells work together to keep a person's blood healthy. Which of the following BEST classifies blood?

A.
organ

B.
tissue

C.
organism

D.
organ system

B. tissue

B. Tissue

To determine the best classification for blood, we need to understand the nature of blood and its components. Blood is a vital fluid that circulates in our bodies, supplying oxygen and nutrients to tissues and removing waste products. Let's examine the given options:

A. Organ: An organ is a distinct structure within a living organism that performs a specific function or multiple functions. While blood is essential for proper bodily function, it does not qualify as an organ because it is not a distinct structure.

B. Tissue: Tissue refers to a group of cells working together to perform a specific function. Blood consists of various types of cells, including red blood cells, white blood cells, and platelets. These cells act together to carry out vital functions such as oxygen transport, immune response, and clotting. Thus, blood is best classified as a tissue.

C. Organism: An organism is a complete living entity that possesses all the essential characteristics of life. Blood is a component of an organism's body, and while it is crucial for the organism's survival, it does not classify as an organism itself.

D. Organ System: An organ system is a group of organs that work together to perform specific functions within an organism. Blood is not an organ system but rather a component of the circulatory system, which includes the heart, blood vessels, and blood itself.

Based on the above analysis, the best classification for blood among the given options is B. Tissue.