the prefix "mono" means one, and you hear it in words like "monogamy," which means attachment to one single partner, or "monologue," which is when one actor in a show or play has an ongoing speech.
so, with that in mind, monopoly will relate to one company rather than many.
(also: for future reference, "poly-" is the prefix for many, so the absence of that could have thrown you off of the second answer as well)
