Explanation:
Original sentence:
"For all he cared, his parent might have been dead for years."

This is a common English expression. The phrase "for all he cared" means he was completely indifferent—he did not care at all.

So the full sentence means:
He was so indifferent that, even if his parents had died long ago, it wouldn’t have mattered to him.

analyzing the options:
A. although concerned about his parent, he had been unable to find out if they were alive
— This implies he cared but was just unsure.
Incorrect

B. He cared for no one except his parents, and it was possible that they had died long before
— This suggests he actually cared for his parents, which contradicts the original.
Incorrect

C. he did not care whether his parents were alive or had died long before
— This matches the meaning exactly.
Correct

D. He was concerned about everyone not just his parents, who might have died long before
— Suggests he cared about many people including his parents. Doesn’t reflect indifference.
Incorrect

E. He cared very much for his parents even though they were dead
— This is the opposite of what the sentence implies.
Incorrect