To determine how much the water in the canteen should change every 10 minutes, we first need to understand the rate at which hikers should drink water.
The recommended amount is 24 ounces per hour. Since there are 60 minutes in an hour, we can find the rate per minute:
\[ \text{Rate per minute} = \frac{24 \text{ ounces}}{60 \text{ minutes}} = 0.4 \text{ ounces per minute} \]
Next, to find out how much water should be consumed in 10 minutes, we multiply the rate per minute by 10:
\[ \text{Amount in 10 minutes} = 0.4 \text{ ounces per minute} \times 10 \text{ minutes} = 4 \text{ ounces} \]
Therefore, the amount of water in the canteen should decrease by 4 ounces every 10 minutes.
So the correct response is:
4 ounces.