Which of the following can a joint tenant not do?

Joint tenants have equal shares in the ownership of a property and all have the right to use and possess the property. Since they have equal shares, they also have the right to sell, use, and mortgage their share without the consent of the others. However, joint tenants cannot pass their share of the property to heirs through a will. Instead, when a joint tenant dies, their share is automatically transferred to the remaining co-owners. This is known as the "right of survivorship" and is a key feature of joint tenancy. Therefore, option C, willing his share to his heirs, is not something a joint tenant can do.