What is the fluid part of the blood and lymph that carries food and secretions to the cells and carbon dioxide from the cells?

The fluid part of the blood and lymph that is responsible for carrying essential nutrients and secretions to cells, as well as transporting carbon dioxide away from cells, is plasma. Plasma is primarily composed of water, proteins, electrolytes, gases, and nutrients, which together perform several critical functions in the body. It provides a medium for transporting these substances throughout the circulatory system, ensuring that cells receive the materials they need to function while also facilitating the removal of waste products like carbon dioxide.

While serum and lymph do play roles in the body's fluids, they do not encompass the full range of functionalities attributed to plasma. Serum is the component of blood that remains after clotting has occurred and does not have the clotting factors found in plasma. Lymph serves important immune system functions and also transports fats and waste, but it operates separately from the bloodstream and does not carry the same range of nutrients and gases essential to cellular functions as plasma does. Thus, plasma is the most accurate answer in the context of the question.