He is leaving the country for a good.

The sentence "He is leaving the country for good" is an idiom. An idiom is a phrase where the meaning isn't obvious from the individual words.

The phrase "for good" doesn't mean "for a good reason" or "for a charitable purpose."

Instead, "for good" means permanently or forever.

Therefore, the sentence implies that the person is leaving the country and will not return.

Let's look at the options:

Option A: "for better prospects" suggests he's leaving for opportunities.
Option B: "for a good cause" means he's leaving for a worthy reason.
Option C: "forever" accurately reflects the meaning of "for good".
Option D: "for others' good" implies he's leaving to benefit others.

Only option C correctly explains the idiom.