Answer: C Labor
In economics, labor refers to physical and mental effort by human beings that is required to produce goods and services in an economy.
Labor refers to all workers who offer their services in exchange for money i.e. they work for gain.
Labor also refers to all the people who work, but don't accumulate and provide the resources for starting a business and do not bear the risk associated with the business.
In the question above, Alexandra owns the business and pays the part-time book-keeper for his services. So, the book keeper represents labor.