To find out how many boxes were packed per minute, divide the total number of boxes by the total time in minutes.
The volunteers packed 50 boxes in 20 minutes:
\[ \text{Boxes per minute} = \frac{\text{Total boxes}}{\text{Total time in minutes}} = \frac{50 \text{ boxes}}{20 \text{ minutes}} = 2.5 \text{ boxes/minute} \]
So, the correct response is 2.5 boxes/minute.