Answer: c. a microeconomist.
Explanation;
Professor Green writing books about labor laws when taken in isolation is not enough proof that he is a microeconomist because even though labor markets and laws fall mostly under microeconomics, they should also be viewed from a macro level as well considering how unemployment affects the economy as a whole.
However, if Professor Green teaches environmental economics as they indeed do, Green is probably a micro-economist because Environmental economics traditionally falls under Microeconomics.