Learn Danish: Vi fandt kirkebogen i hvilken dåben er registreret. - We found the parish register in which the baptism is registered.

The relative pronoun must match the gender of the noun it refers back to. Here, it refers to kirkebogen (the parish register). Since en bog is a common-gender noun, we use hvilken. If the noun were neuter, we would use hvilket.

Is i hvilken commonly used in everyday conversation?

Not usually. The construction preposition + hvilken (like i hvilken) is very formal and mostly found in written Danish, like literature, official documents, or academic texts. In everyday speech, you would use som and move the preposition to the end of the clause: som dåben er registreret i.

Why use er registreret instead of bliver registreret for the passive voice?

We use er + past participle to describe a state. The baptism is already recorded and sits there in the book. If you used bliver registreret, it would mean the action is happening right now: the baptism is being registered by someone holding a pen.

Can I use hvor instead of i hvilken in this sentence?

Yes, that is a great alternative! You could say hvor dåben er registreret (where the baptism is registered). This is slightly less formal than i hvilken but still perfectly correct in both written and spoken Danish when referring to a physical or figurative location like a book.