Reageer je op mijn e-mail? (Will you reply to my email?)
reageren takes "op" (reageren op = to react to). Example: Reageer je op mijn e-mail? (Will you reply to my email?)
Reageer je op mijn e-mail? — Will you reply to my email? 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.