To find the unit rate for how long it took Carla to run one mile, we need to divide the total time by the total distance.
Carla took 36 minutes to run 4 miles.
To find the time it took her to run one mile, we can calculate:
\[ \text{Time per mile} = \frac{\text{Total time}}{\text{Total distance}} = \frac{36 \text{ minutes}}{4 \text{ miles}} = 9 \text{ minutes/mile} \]
So, the correct unit rate is 9 minutes/mile.