What is barter? a. a means for comparing the values of goods and services b. something that keeps its value if it is stored rather than used c. anything that serves as a medium of exchange, a unit of account, and a store of value d. the direct exchange of one set of goods or services for another

The answer you're looking for is D, The direct exchange of one set of goods or services for another

The correct option is D.
In a barter system, goods and services are exchanged for other goods and services without making use of any medium of exchange such as money. For instance, five big tubers of yam may be exchange for a chicken, if the exchange condition is acceptable to the contracting party. The exchange typically take place immediately and not on a future date.

Ibelieve that the answer is (a).