Aperson can achieve optimum daily nutrition by
a. eating a variety of healthy foods.
b. drinking at least six glasses of juice.
c. excluding all fat from his or her diet.
d. eating large amounts of lean protein.

Optimum daily nutrition can be achieved by A - eating a variety of healthy foods.

The other options, while they can be useful as well, don't nearly produce the same effects as does eating a variety of healthy foods. The reason for this is that a very varied diet in different foods is the key to a healthy life, while emphasizing only nutrient group is the wrong way to approach optimum daily nutrition. 

Ithink its c hope it : )