The correct answer is A. flat rate tariff.
In a flat rate tariff, the consumer pays the same amount for electricity regardless of how much they use. This means that they have to pay a lesser fixed charge than they would in a two part tariff or a maximum demand tariff.
In a two part tariff, the consumer pays a fixed charge plus a variable charge based on how much electricity they use. This means that they have to pay a higher fixed charge than they would in a flat rate tariff, but their variable charge will be lower if they use less electricity.
In a maximum demand tariff, the consumer pays a fixed charge plus a variable charge based on the maximum amount of electricity they use in a given period of time. This means that they have to pay a higher fixed charge than they would in a flat rate tariff, and their variable charge will be higher if they use more electricity.
Therefore, the consumer has to pay the least amount of fixed charges in a flat rate tariff.