Choose the right word to finish the overly honest real estate agent's listing.

This is the creepy Victorian mansion in _____ the previous owners claimed to see flying teacups.

The correct answer is which.

When a relative pronoun directly follows a preposition (like "in", "on", or "about"), we must use which for things or whom for people. We cannot use that directly after a preposition (e.g., "in that" is incorrect here), and "in where" is grammatically invalid.