How does food with proper nutrients benefit you?

a.) it provides energy and fights infections.
b.) it serves as a building block of protein.
c.) it maintains healthy muscles and cells.
d.) it regulates essential chemicals in the body.

The answer to this question would be  A.) It provides energy and fights infections.

The nutrient is a substance that needed to maintain life. This included carbohydrate which gives most energy, an amino acid that makes protein and used for making cells, or vitamin for reaction coenzyme. 
The option in this question is not clear and I think all of them can be said as a benefit of nutrient. The option A is about carbohydrate, option B and C about protein/amino acid and option C about vitamin.
But the most vital need for an organism is energy, so I think A will be the best option if you need to choose only one.

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