Final answer:
The barter system best illustrates the double coincidence of wants, which is a situation where two people each want goods or services that the other can provide. So, the correct answer is option a.
Step-by-step explanation:
In an economy without money, an exchange between two people would involve a double coincidence of wants, a situation in which two people each want some good or service that the other person can provide. The best illustration of the double coincidence of wants is the barter system. In a barter system, goods or services are directly exchanged without the use of money, relying on the mutual needs and wants of the individuals involved.
So, the correct answer is option a.