Answer:
Having one tone.
Step-by-step explanation:
Prefixes are the words that are added to an already existing word to make a new word with an altogether new meaning. Accordingly, the prefix "mono-" is added to the word "tone" to make a new word "monotone".
Now, looking at the given example sentence, the word "monotone" is used to refer to the tone of Mr. Goggin. And from the prefix "mono-" which means single, we can infer that "monotone" means a single, unchanging tone.
Thus, the definition of "monotone" is "having one tone".