This is in the passive voice. Here's why:
* Passive voice occurs when the subject of the sentence is acted upon by the verb. In this case, "A northwest passage to India" is being acted upon by "discovered".
* Active voice would put the actor (the person or thing that discovered the passage) as the subject. For example: "Explorers discovered a northwest passage to India."
Let me know if you'd like more examples or explanations!