Learn Russian: Он обращается к нам с вопросом.

No, it is active! While adding -ся to an imperfective verb often makes it passive (like дом строится - the house is being built), many Russian verbs naturally end in -ся and have an active or reflexive meaning. Обращаться literally means "to turn oneself to" or "to address," so the subject (он) is actively performing the action.

Why do we need the preposition к before нам?

The verb обращаться requires the preposition к (to / toward) followed by the Dative case to indicate who is being addressed or approached. Нам is the Dative form of мы (we), so к нам means "to us."

Why use обращается instead of a verb of motion like приходит (comes)?

The English translation "comes to us" is figurative here. Обращаться does not mean physical walking. It means "to turn to," "to appeal to," or "to address." It implies he is directing his inquiry at us, rather than necessarily physically walking toward us.

Why is it с вопросом and not just с вопрос?

The preposition с when it means "with" (as in "along with" or "having something") always takes the Instrumental case. For the masculine noun вопрос, you add the ending -ом to get вопросом.