Final answer:
The seller is relieved of the liability when the buyer assumes the mortgage through novation, which creates a new contract and releases the original borrower from responsibility.
Step-by-step explanation:
When a buyer assumes the mortgage, the seller can be relieved of the liability through a process called novation. This involves creating a new contract that replaces the old one, effectively releasing the seller from any further responsibility. The other options listed, being subject to mortgage, deed of release, and defeasance, do not pertain to the transfer of liability in the context of a mortgage assumption. A deed of release is used when the debt is paid off, and defeasance is a clause in a mortgage that provides for the voiding of the mortgage upon fulfillment of certain conditions. Meanwhile, subject to mortgage means the property is being sold with the mortgage in place, but this does not release the original borrower from liability.