Final answer:
Robert's adjusted basis for the land is $450,000.
Step-by-step explanation:
Robert's adjusted basis for the land would be $450,000 (option c). In a joint tenancy, when one tenant dies, the surviving tenant receives a step-up in basis for the deceased tenant's share of the property. This means that Robert's basis for Diane's share of the land is adjusted to its fair market value at the time of her death, which is $450,000. Therefore, Robert's adjusted basis for the land is $450,000.