Choose the correct meaning of the underlined phrase from the options given below:

The toughest nut to crack is the profound philosophical question: What is life?

difficult to resolve

The correct answer is Option (1) → difficult to resolve

The phrase "the toughest nut to crack" is an idiom that means the most difficult problem or challenge to solve or overcome. It is often used to describe a situation or question that is particularly complex or perplexing. In the context of the sentence provided, it refers to the profound philosophical question "What is life?" which is considered a challenging question to answer due to its depth and complexity. Therefore, Option 1, "difficult to resolve," accurately captures the meaning of the phrase in this context.

Here's why the other options are incorrect:

* **Irritating problem:** While challenging, the question of life's meaning isn't necessarily framed as an annoyance in the sentence.
* **Never-ending issue:** The question itself might be ongoing, but the phrase "nut to crack" suggests finding a solution, not something perpetual.
* **A tall hurdle:** A hurdle suggests something to overcome, but "crack" implies solving or understanding something complex.

"Toughest nut to crack" is a metaphor. A nut is something hard to open, and similarly, the question of life's meaning is a complex problem that is difficult to solve or understand definitively. Therefore, option 1 best captures the intended meaning.