Correct Answer: To understand the implied meaning

Solution : The fourth option is the correct answer.

Reading between the lines refers to the act of discerning or grasping a meaning that is not explicitly stated or expressed. It involves understanding the implied or hidden message, often conveyed through subtle nuances or indirect language.

Therefore, to understand the implied meaning accurately encapsulates the essence of the idiom and is the most fitting interpretation.

Therefore, the correct answer is," to understand the implied meaning".