Final answer:
The term 'y' in the expression '9x + 4y' represents the number of grams of carbohydrates and proteins in a food item, with each gram contributing 4 calories to the total caloric content.
Step-by-step explanation:
When analyzing the expression given for the total number of calories in a food item, which is 9x + 4y, the term y represents the number of grams of carbohydrates and proteins in that food item. The reason for this is based on the information provided: a gram of fat has about 9 calories, and both carbohydrates and proteins contain about 4 calories per gram. The expression is thus a way to calculate the total caloric content of a food item by multiplying the grams of fat by 9 and the grams of carbohydrates and proteins by 4, and adding them together.