What does intracellular fluid primarily consist of?

Intracellular fluid primarily consists of cytoplasm, which is the gel-like substance found within the cell membrane. This fluid contains water, salts, and various organic molecules, and it serves as the medium for biochemical reactions essential to cell function. Notably, the cytoplasm encompasses all contents of the cell outside the nucleus, including organelles and other cellular components.

The other choices represent fluids found in different parts of the body or serving different functions. Interstitial fluid is the fluid that fills the spaces between cells and is part of the extracellular fluid. Blood plasma is the liquid component of blood that carries cells and various substances; it is also part of the extracellular fluid. Serous fluid is the lubricating fluid found between serious membranes, such as in the pleural or peritoneal cavities, which also does not comprise the internal environment of the cell. Thus, cytoplasm, being the core component of intracellular fluid, is the correct answer.