Ze protesteren tegen het plan. (They protest against the plan.)
protesteren takes "tegen" (protesteren tegen = to protest against). Example: Ze protesteren tegen het plan. (They protest against the plan.)
Ze protesteren tegen het plan. — They protest against the plan. The preposition tegen 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.