All living cells are composed of which substance?

The correct choice is protoplasm because it refers to the living substance within a cell, which includes the cytoplasm and the nucleus. This term encompasses all the essential components necessary for cell function and activity, such as organelles, cytosol, and various macromolecules. Protoplasm is a crucial element in defining what makes a cell alive, as it is where most of the cellular processes occur.

Cytoplasm, while also a key component of cells, is specifically the gel-like substance that fills the cell and surrounds the organelles, but it does not include the nucleus. Mitochondria are important organelles responsible for energy production, but they are not the substance that makes up all living cells. Cellulose, on the other hand, is a structural component found primarily in plant cells and is not present in all living cells. Therefore, protoplasm is the most comprehensive answer as it encompasses all living cellular components.