The correct answer is B, Demand.
Demand is the consumers' ability and desire to buy good and services.
Demand is the consumer's willingness to buy good or service by paying money for that. When people want to buy something by paying for it, it means they are demanding that product or service.
It is the basic concept in economics that when demand for a product or service increases, the price of that products also increases. Similarly, when demand of a product or service decreases, the price of that product or service also decreases.