The correct interpretation of the slope in the linear model \( y = 17.5x + 480 \) is:
Each additional day is associated with an additional 17.5 grams of mass.
In this equation, \( y \) represents the mass of the Boxer puppy, and \( x \) represents the age in days. The slope (17.5) indicates that for each additional day in age, the mass of the puppy is expected to increase by 17.5 grams.