Final answer:
The determinants of demand include desire, willingness, and the ability to pay. Without the ability to pay for a good or service, there is no effective demand, even if there is a want or need.
Step-by-step explanation:
Understanding how business is done in the U.S. economy often comes down to grasping the concepts of supply and demand. The question asks about the determinants of demand, which include having a desire and willingness, alongside another crucial factor. For demand to be effective, consumers must have the ability to pay for the goods or services they desire. If you do not have the means to purchase something, no matter how much you may want or need it, you do not have effective demand for it. Therefore, the correct option for completing the sentence 'The determinants of demand include having a desire, willingness, and...' would be Ability.