When discussing economics, factors of production refer to the resources used to create goods and services. Among the options provided:
- A. A sofa produced by a furniture manufacturer – This is an output of production, not a factor.
- B. 20 shares of Microsoft stock – Stocks represent ownership in a company, not a direct factor of production.
- C. The janitor at the local elementary school – This represents labor, which is indeed one of the factors of production.
- D. 500 in cash – Cash is a financial asset, but it is not a direct factor of production.
Therefore, the correct answer is C: the janitor at the local elementary school, as labor is a vital component in the production process.