Answer:
The correct answer is letter "B": The price will not increase but firms will increase the quantity supplied to promote the social interest.
Step-by-step explanation:
Perfectly competitive markets are characterized by having companies offering an undifferentiated product, being price takers because firms posses a small market share which does not allow them to have a major influence in the price, and by free entry and exit of competitors.
Then, if there is a shortage of clean drinking water in a local market that is perfectly competitive, the shortage would not last much since new producers would enter the market to process water so it can be offered purified. As drinking water is a basic good, the number of organizations entering the market is likely to be substantial.