Jesus said it would be better to have what hung around one's neck than to make a little one stumble?

Answer: A millstone

Mark 9:42 says it would be better to be thrown into the sea with a millstone around the neck.

“But if anyone causes one of these little ones who believe in Me to stumble, it would be better for him to have a large millstone hung around his neck and to be thrown into the sea.”— Mark 9:42 (BSB)