The correct answer is C. Different elasticity of demand.
A discriminating monopoly is a monopoly that can charge different prices to different consumers for the same good or service. This is possible if the monopoly can prevent consumers from reselling the good or service to each other.
For a discriminating monopoly to be profitable, the demand for the good or service must be elastic in some markets and inelastic in other markets. This is because the monopoly can charge a higher price in the inelastic markets and a lower price in the elastic markets.
If the demand for the good or service is equal in all markets, then the monopoly cannot charge different prices without losing customers. This is because consumers would simply buy the good or service in the market where the price is lower.
Rising and declining cost curves are not relevant to the question of whether or not a discriminating monopoly is possible. The cost curves only affect the amount of profit that the monopoly can make, not whether or not it can charge different prices.