To find the unit rate of how many minutes Liam took to travel one mile, we can divide the total time by the total distance.
Liam took 15 minutes to ride 3 miles.
So, to find the time taken per mile:
\[ \text{Time per mile} = \frac{\text{Total time}}{\text{Total distance}} = \frac{15 \text{ minutes}}{3 \text{ miles}} = 5 \text{ minutes/mile} \]
Thus, the correct unit rate is D) 5 minutes/mile.