Help the landlord address the correct tenants in his email by choosing the correct phrase.

The landlord finally agreed to fix the broken radiator in ________ shared apartment.

The correct answer is Arthur and Merlin's.

When two or more people own the same single item together (joint possession), we only add the apostrophe and "s" to the last name in the group. Because Arthur and Merlin share one apartment, it is "Arthur and Merlin's apartment."

If they each had their own separate apartment, you would use "Arthur's and Merlin's apartments."