Hij is boos op zijn broer. (He is angry at his brother.)
boos takes "op" (boos op = angry at). Example: Hij is boos op zijn broer. (He is angry at his brother.)
Hij is boos op zijn broer. — He is angry at his brother. The preposition op is fixed, so it doesn't change with the sentence.
Fixed prepositions come up all over the A2 exam. The platform drills them in real exam tasks, AI-graded across all five DUO sections.