Which of these tribes was to stand on Mount Ebal to pronounce the curse?

Answer: Reuben

Simeon, Levi, Judah, Issachar, Joseph and Benjamin stood on Gerizim to bless; Reuben, Gad, Asher, Zebulun, Dan and Naphtali on Ebal.

““When you have crossed the Jordan, these tribes shall stand on Mount Gerizim to bless the people: Simeon, Levi, Judah, Issachar, Joseph, and Benjamin. And these tribes shall stand on Mount Ebal to deliver the curse: Reuben, Gad, Asher, Zebulun, Dan, and Naphtali.”— Deuteronomy 27:12-13 (BSB)