A primary function of carbohydrates in the body is to provide:

The primary function of carbohydrates in the body is to provide fuel for cell metabolism. Carbohydrates are essential macronutrients that the body converts into glucose, which is the preferred energy source for most cells, especially in the brain and during physical activity. When carbohydrates are consumed, they undergo digestion and are broken down into simple sugars, primarily glucose. This glucose is then utilized by cells to generate adenosine triphosphate (ATP), the energy currency of the cell, through processes like glycolysis and cellular respiration.

While glucose is indeed a form of fuel that cells use, it's important to note that carbohydrates also play a role in sparing proteins from being used for energy, allowing them to fulfill their primary functions, such as building and repairing tissues. Therefore, this makes the provision of energy a critical role of carbohydrates, underscoring why the correct answer highlights their importance in cell metabolism.

In contrast, immunity and red blood cell production are influenced by other nutrients and systems in the body, and while glucose and fats can provide energy, the direct role of carbohydrates is more clearly defined as serving as a primary fuel source. Thus, option C accurately captures the essential role of carbohydrates in supporting cellular energy needs.