The correct answer is: B. The teacher asked the boys what they understood by this phrase.
The teacher is asking the boys to explain what they understand by the phrase that she has just said. The phrase is “Boys, what do you understand by this phrase?”. The teacher is using the word “boys” to address the group of students in the class. The word “understand” means to comprehend or to know the meaning of something. The word “phrase” means a group of words that form a unit of meaning. The teacher is asking the boys to explain what they understand by the phrase that she has just said.
The other options are incorrect because they do not accurately reflect what the teacher is saying. Option A is incorrect because the teacher is not asking the boys what they understood by that phrase. Option C is incorrect because the teacher is not asking the boys what they understand by this phrase. Option D is incorrect because the teacher is not asking the boys what they understand by that phrase.