✅ The correct answer is "whose" because it is the possessive relative pronoun that refers to "the ancient cathedral." "Whose" can correctly be used for both people and inanimate objects to show possession, indicating that the gargoyles belong to the cathedral. It functions as a possessive determiner within the relative clause, modifying "gargoyles."

❌ Option A, "which its," is grammatically incorrect due to redundancy. "Which" is a relative pronoun, and "its" is a possessive pronoun; using both creates an awkward and ungrammatical construction.

❌ Option B, "of which," can express possession for inanimate objects, but it does not fit the structure of the sentence here. To use "of which," the phrase would typically be structured as "the gargoyles *of which*" or "of which the gargoyles," not "of which gargoyles."

❌ Option D, "that its," is incorrect for two main reasons. First, like "which its," it suffers from redundancy with both "that" and "its." Second, "that" is generally not used to introduce non-restrictive clauses (which are set off by commas and provide additional, non-essential information), whereas "whose" is perfectly suitable in such contexts.