Choose the most appropriate meaning of the underlined phrase.
He's still keeping his chin up despite all his health problems.

The correct answer is "To stay cheerful during difficult times".

Explanation:
The idiom "keep one's chin up" is a common English expression that means to remain hopeful, positive, and cheerful in a difficult, discouraging, or challenging situation.
In the given sentence, "He's still keeping his chin up despite all his health problems," the context of facing health problems makes it clear that the person is trying to remain optimistic and cheerful despite the adversity.

Let us look at why the other options are incorrect:
- "To keep one’s chin clean and moist" is a literal, non-idiomatic interpretation of the words and is incorrect.
- "To stay safe during difficulty" and "To save others during difficult times" do not match the meaning of the idiom, which specifically refers to one's mood, attitude, or emotional state (staying cheerful/optimistic).