Final answer:
In the sentence presented, the commas are used to enclose a parenthetical expression, which is additional information that could be removed without altering the meaning of the sentence. Comma usage here signals a brief pause and provides context without the emphasis a dash would add.
Step-by-step explanation:
The comma in the sentence "Who, in your opinion, won the battle of Waterloo?" is used as an interrupter in the form of a parenthetical expression. This is a type of nonessential information which adds additional details or context to the sentence but does not alter its basic meaning. The parenthetical expression could be omitted, and the sentence would still be grammatically correct and its meaning clear, which is a key characteristic of nonessential or parenthetical information.
It's worth noting that in some situations, a dash or parentheses might also be used to set off nonessential information. However, the use of commas here indicates a slight pause and less emphasis than a dash would convey. If the commas were replaced with dashes as in "Who—in your opinion—won the battle of Waterloo?", the interruption would be more pronounced and add dramatic effect.
Commas also serve various other functions, such as separating items in a list, marking off nonessential information, after introductory elements, and before coordinating conjunctions. The key is that commas typically indicate a pause in the sentence, signaling the reader that nonessential or additional information is being presented.