Answer:
The third option, "His books, in my opinion, are so similar that it's difficult to distinguish one plot from the other." Is the correct answer.
Step-by-step explanation:
This sentence has an appositive and the commas are being used correctly.
An appositive is basically a noun or a noun phrase that further identifies another noun right next to it.
Rule of thumb:
- Use commas around an appositive when it is NOT essential to the sentence or information.
- One word appositivies (generally) don't need commas.
- If the appositive gives weight or meaning to a sentence, then you don't need to put it around the appositive.