The correct answer is - B. consumers/producers.
Within an economy, of any type, there's an exchange of goods and services between the producers and consumers. In order for something to get on the market someone has to produce it or provide it, be it a good or a service, and that good or service is used by someone, thus the consumers.
While the producers make certain thing, the consumers buy it or exchange it. Depending on the type of economy, it can be money in exchange for a good or service (which is the dominant type by far), or it can be a good for good, or a service for service, which has been a dominant type in the past, but not anymore.