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