Answer:
keep buying a product until marginal benefit equals price
Step-by-step explanation:
A rational consumer would continue to consume a product until the marginal benefit of the last unit consumed equal marginal cost. At this point, utility is maximised.
For example, if the price of a bottle of water is $4. The utility you derive from the first bottle is 6. So you consume one more bottle, the utility you derive from the second bottle is 5. you buy a third bottle. The utility you derive from the 3rd bottle is 4. At this point utility is maximised and you should stop consuming more water
If you consume a 4th bottle, the utility you would derive from it would be 3 utils. This doesn't make sense because you are paying more for the bottle when compared to the utility you would derive from it