Final answer:
The element that is not considered a macronutrient from the options provided is hydrogen (a). Macronutrients are needed in large amounts and include elements like carbon, nitrogen, and minerals such as calcium and magnesium.
Step-by-step explanation:
Macronutrients in Biology
Macronutrients are generally required in large amounts by organisms and are crucial for the maintenance and functioning of biological systems. These include elements like carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Macronutrients also include certain metals such as potassium, magnesium, calcium, and sodium. Among these, the one that is not considered a macronutrient is hydrogen (a). Hydrogen is indeed essential as it is a part of water molecules and many organic compounds within cells, but it is not classified as a macronutrient.