Answer:
a. There must be at least two parties
Step-by-step explanation:
Option Includes: a. There must be at least two parties, b. Each party is free to accept or reject the exchange offer c. Each party has something that might be of value to the other party, d. Each party believes it is appropriate or desirable to deal with the other party.
Since Cristobal has listed its Ford F150 truck on eBay for weeks now but no one has so far contacted him about the truck, it means that there is no exchange taking place because of the absence of the other party. As there must be at least two parties for the condition of exchange to sell the product which is found absent here.
All the other options such as desirable item to deal or something valuable for both to offer will be relevant only when the other part approaches cristobal in response of his offer and then discuss the terms to see whether the deal meets desired needs and whether the deal will be valuable for both will be also determined only after two parties discuss the terms.