To find the rate of change of the average height of the trees with respect to the number of years since they were planted, we can calculate the slope of the line that passes through the two points given: $(3, 24)$ and $(11, 80)$.

The formula for the slope (rate of change) $m$ between two points $(x_1, y_1)$ and $(x_2, y_2)$ is:

$m = \frac{y_2 - y_1}{x_2 - x_1}$

Substituting the given values:

$m = \frac{80 - 24}{11 - 3} = \frac{56}{8} = 7 \text{ feet per year}$

So, the rate of change of the average height of the trees with respect to the number of years since they were planted is 7 feet per year.

The correct answer is:

C. 7 feet per year

Would you like details on the calculation process, or do you have any questions?

Here are 5 related questions you might find useful:

1. How do you interpret the rate of change in a linear relationship?
2. What is the significance of the slope in the context of real-life situations?
3. How can you determine if two quantities have a linear relationship?
4. What does it mean if the rate of change is zero?
5. How would the problem change if the growth rate was not constant?

Tip: Always check the units in word problems to ensure your calculations are correct.