Explanation:

The correct answer is option 'D', which is "at".

Reasoning:

The word "at" is used to indicate a specific point or time. In this sentence, the teacher set some homework at the end of the lesson. This means that the homework was given at the specific time when the lesson was about to finish or had just finished.

Usage of Prepositions:

Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. In this case, the preposition "at" is used to indicate the specific time or point when the homework was given.

Examples of Prepositions:

- "The party is at 8 PM." (The preposition "at" indicates the specific time of 8 PM)
- "She arrived at the train station." (The preposition "at" indicates the specific location of the train station)

Alternative Options:

Let's examine the other options to understand why they are incorrect:

- Option 'A' ("about") is not the correct answer because "about" is used to indicate a general topic or subject, not a specific time or point. It does not fit the context of the sentence.
- Option 'B' ("in") is not the correct answer because "in" is used to indicate a general period of time, not a specific time or point. It does not fit the context of the sentence.
- Option 'C' ("of") is not the correct answer because "of" is used to indicate possession or origin, not a specific time or point. It does not fit the context of the sentence.

Conclusion:

In conclusion, the correct answer is option 'D' ("at") because it accurately indicates the specific time or point when the teacher set the homework at the end of the lesson.