Psalm 2:7: the LORD's decree says, "You are My ___; today I have become Your Father."

Answer: Son

In Psalm 2 the LORD installs His King on Zion and declares, "You are My Son" (KJV: "this day have I begotten thee").

“I will proclaim the decree spoken to Me by the LORD: “You are My Son; today I have become Your Father.”— Psalm 2:7 (BSB)