On which mountain was the blessing to be proclaimed when Israel entered the land?

Answer: Mount Gerizim

Moses commanded that the blessing be proclaimed on Mount Gerizim and the curse on Mount Ebal.

“When the LORD your God brings you into the land you are entering to possess, you are to proclaim the blessing on Mount Gerizim and the curse on Mount Ebal.”— Deuteronomy 11:29 (BSB)