At Sinai, what did God say Israel would be if they obeyed and kept His covenant?

Answer: His treasured possession

If Israel obeyed and kept the covenant, they would be God's treasured possession out of all nations, for the whole earth is His.

“Now if you will indeed obey My voice and keep My covenant, you will be My treasured possession out of all the nations—for the whole earth is Mine.”— Exodus 19:5 (BSB)