Let this be an example . . . . . . . . the troublemakers. A. about B. for C. against D. to

The correct answer is: to.

The word “to” is used to indicate the purpose or goal of something. In this case, the speaker is using the word “to” to indicate that the example is being given in order to teach the troublemakers a lesson.

The other options are incorrect because they do not accurately reflect the speaker’s intention. The word “about” is used to introduce a topic or subject. The word “for” is used to indicate the reason or purpose of something. The word “against” is used to indicate opposition or disagreement.

Here is a more detailed explanation of each option:

• About: The word “about” is used to introduce a topic or subject. In this case, the speaker is not introducing a topic or subject, but rather giving an example.
• For: The word “for” is used to indicate the reason or purpose of something. In this case, the speaker is not giving the example for a reason, but rather to teach the troublemakers a lesson.
• Against: The word “against” is used to indicate opposition or disagreement. In this case, the speaker is not opposing or disagreeing with anything, but rather giving an example.

I hope this explanation is helpful!