This question requires your understanding of the word elucidate. Essentially, this means to make something clear, or to clarify what is said. I would also imagine that you are being asked to pick a phrase that is not meant literally but figuratively. Thus, you must identify the purpose of the phrase, thereby making it clear.
For example, often English speakers will say that "it is raining cats and dogs." When we do say this we do not literally mean that cats and dogs are falling from the sky. We mean the rain is coming down so hard. We exaggerate by substituting a small, sometimes easily provoked animal for the pelting rain. Cats and dogs cannot fall from the sky in the form of rain. But we can often feel that rain is nagging us and inconveniencing us like small animals can. This paragraph is your elucidation. It makes clear how and why the phrase is used.
You might consider looking up other idioms or colloquialisms in the English language. You could uncover other phrases to use and research.