When money is paid or goods delivered by mistake, the recipient must return them under the law of restitution. There is no transfer of ownership - the legal title remains with the original owner who mistakenly paid. Option B correctly states the obligation to repay or return. Option A is incorrect - mistake doesn't transfer ownership. Option C is wrong - you cannot sell what doesn't belong to you. Option D incorrectly suggests voluntary return is optional; it's a legal obligation. The principle prevents unjust enrichment.