In Moses' final blessing, which tribe is "a lion's cub, leaping out of Bashan"?

Answer: Dan

Deuteronomy 33:22: "Concerning Dan he said: 'Dan is a lion's cub, leaping out of Bashan'" (KJV: "a lion's whelp").

“Concerning Dan he said: “Dan is a lion’s cub, leaping out of Bashan.””— Deuteronomy 33:22 (BSB)