Final answer:
The correct unit to describe the rate of change or slope of the function is dollars per chicken.
Step-by-step explanation:
The correct unit to describe the rate of change or slope of the function is dollars per chicken. In this case, the slope of the function represents how the charge in dollars changes for every one unit increase in the number of chickens. Since the slope of f(x) = 2x + 40 is 2, it means that for every additional chicken, the charge increases by $2. Therefore, the correct unit is dollars per chicken.