When the LORD asked Cain where his brother Abel was, what did Cain ask in reply?

Answer: "Am I my brother's keeper?"

Cain, who had killed Abel in the field, answered, "I do not know!" and asked, "Am I my brother's keeper?"

“And the LORD said to Cain, “Where is your brother Abel?” “I do not know!” he answered. “Am I my brother’s keeper?””— Genesis 4:9 (BSB)