Final answer:
Oxygen (A) is never a limiting nutrient in cells because it is abundantly available in the environment for processes like cellular respiration, unlike other elements such as nitrogen, phosphorus, and sulfur which may limit biological processes due to their less availability and high demand for macromolecule synthesis.
Step-by-step explanation:
The element that is never a limiting nutrient in cells is oxygen (A). This is because oxygen is abundant in the environment, especially in the atmosphere, and is readily available for use in cellular respiration, where it serves as the final electron acceptor. In comparison, elements like nitrogen, phosphorus, and sulfur often limit biological processes because they are required in large amounts for the synthesis of macromolecules and are not as readily available in the environment. For example, plants can only utilize nitrogen when it has been converted from atmospheric nitrogen (N₂) to an organic form by nitrogen-fixing microorganisms. Carbon and hydrogen are part of many organic compounds and water, and while hydrogen is the most abundant element in cells, it is not typically a limiting nutrient because of its prevalence in water molecules.