If the distribution of water is a natural monopoly, then:__________.
a. a single firm cannot serve the market at the lowest possible average total cost.
b. allowing for competition among different firms in the water-distribution industry is efficient.
c. average cost increases as the quantity of water produced increases.
d. multiple firms would likely each have to pay large fixed costs to develop their own network of pipes.

The initial investment is very high, therefore, the more firms competing will only increase the required investments and fixed costs associated with them, e.g. depreciation, maintenance. That is why the lowest average costs is generally achieved when only one firm serves this type of market.

Option b is wrong because:

A natural monopoly exists because it is extremely difficult for two or more competing firms to exist. Generally the required investment is very high, and the revenues are not large enough to allow two or more firms to compete.

Option c is wrong because:

Utilities require large initial investments, but once they are set up, the production costs are very small. I.e. the fixed costs are more relevant than the variable costs. Average production costs as decrease as the quantity produced increases.