Actually, nutrition is none of those things and all of those things.
Nutrition is the study of how animals turn their food into living body tissue.
Nutritionists are really not as much interested in what a deer eats as what nutrients are in that food. In order to ensure that a deer herd has an adequate diet, we need to know the nutrient requirements of deer, how much food is available, and the nutrient content of that food. Unfortunately, nutrition is not an exact science. Probably no other species of wildlife have been studied as much as the white-tailed deer, and yet we still do not know enough about their nutrient requirements. Complicating matters further are the facts that food availability, by species and amount, changes seasonally, the nutrient content of foods varies by season, and the deer themselves vary their intake over the year.