Which of the following best describes protoplasm?

Protoplasm is best described as the living material in cells. This encompasses all the components within a cell that are essential for life, including the cytoplasm and the nucleus, where cellular activities and processes take place. Protoplasm is crucial for the functions of life, such as metabolism, growth, and reproduction.

The other options, while they may relate to different biological concepts, do not accurately represent protoplasm. For example, muscle tissue is made up of cells but does not define protoplasm itself. Non-living substances are not composed of living material, which rules out that option as well. Lastly, while blood does contain fluid components, protoplasm specifically refers to the living material within cells, not the fluid that circulates in the bloodstream. Understanding the significance of protoplasm in cellular function highlights its importance in biology and its role in the existence of living organisms.