Whose offering did God accept, Cain's or Abel's?

Answer: Abel's

The Lord looked with favor on Abel and his offering, but not on Cain's.

“while Abel brought the best portions of the firstborn of his flock. And the LORD looked with favor on Abel and his offering, but He had no regard for Cain and his offering. So Cain became very angry, and his countenance fell.”— Genesis 4:4-5 (BSB)