Which of the following is a form of multiple ownership in which each owner possesses an undivided proportional interest in the entire property, although the ownership shares need not be equal?

Tenancy by the entirety is a form of ownership available only to married couples, where both spouses hold an equal and undivided interest in the property. This form of ownership includes rights of survivorship, meaning if one spouse dies, the other automatically inherits the entire property. Unlike tenancy in common, shares in tenancy by the entirety cannot be unequal.

Tenancy at will refers to an arrangement where a tenant occupies the property without a formal lease agreement, allowing either party to terminate the tenancy at any time. It does not involve multiple ownership or shared interests in property but rather a temporary occupancy arrangement. Therefore, it does not fit the criteria of undivided interest among multiple owners.

Joint tenancy is characterized by equal ownership shares among co-owners, along with the right of survivorship. In this arrangement, if one owner dies, their share automatically transfers to the remaining owners. Unlike tenancy in common, joint tenants cannot hold unequal interests in the property, which differentiates it from the correct answer.