Ik wacht op de bus. (I'm waiting for the bus.)
wachten takes "op" (wachten op = to wait for). Example: Ik wacht op de bus. (I'm waiting for the bus.)
Ik wacht op de bus. — I'm waiting for the bus. 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.