Final answer:
Former Mexican citizens living in Texas had a legal claim to land they owned in Texas, in accordance with the Treaty of Guadalupe Hidalgo, despite subsequent disregard for these rights.
Step-by-step explanation:
In the case of John F. McKinney v. Manuel Saviego, there isn't a specific Supreme Court ruling to reference as this case appears to be fictional or not well-known. However, considering the historical context provided, it is relevant to discuss the treatment of former Mexican citizens and their land claims after the Treaty of Guadalupe Hidalgo. This treaty, which ended the Mexican-American War, ensured that Mexicans who stayed in the ceded territories could become American citizens and keep their property. Despite these protections, there was widespread disregard for these rights as evidence by the stripping of land from Mexicans and Mexican-Americans in the following decades.
Based on the historical context and the issues related to the treatment of former Mexican citizens in Texas, the correct answer concerning their legal status would likely align with the treaty's provisions. Thus, the answer to the question is that former Mexican citizens living in Texas:
D. had a legal claim to land they owned in Texas.