The interaction of individual choices

Hilary has opened a new startup company in web design. Within the first month of business, the startup agrees to maintain an accounting firm's website in exchange for someone at the firm doing the startup's tax returns.

Which of the following principles of economic interaction best describes this scenario?

All costs are opportunity costs.

Trade can make everyone better off.

When markets do not achieve efficiency, government intervention can improve overall welfare.

Markets allocate goods effectively.