Final answer:
The opportunity cost of owning and working in your own firm is income earned from working at another occupation. Therefore, the correct option is A.
Step-by-step explanation:
The opportunity cost of owning and working in your own firm would be A. income earned from working at another occupation. Opportunity cost refers to the value of the next best alternative that is forgone when making a choice. In this case, by owning and working in your own firm, you are giving up the potential income that you could have earned from working at another occupation.