When a firm$$s average costs decrease over their entire range of relevant output, it is socially efficient to have one firm supply to the entire market as opposed to multiple firms each producing a smaller amount. Suppose firms in a market produce a homogeneous good and have high fixed costs of production and low, stable marginal costs. In which of the following cases do consumers not benefit from a natural monopoly serving the entire market?

Choice $1$ of $4$:a$.$ When the monopolist is regulated to price at average cost.Choice $2$ of $4$:b$.$ When the monopolist is regulated using marginal cost$-$plus pricing.Choice $3$ of $4$:c$.$ When the monopolist is unregulated.Choice $4$ of $4$:d$.$ When the monopolist is regulated to make zero economic profit.