Which one of the following diseases of the oral mucosa is described as an immovable white patch that has no obvious cause?

Leukoplakia is characterized as an immovable white patch on the oral mucosa that cannot be attributed to any obvious cause, which aligns perfectly with the description provided in the question. This condition is often considered a potentially precancerous lesion, as it can occur due to various factors like chronic irritation, tobacco use, or other local diseases, but the specific irritants or causes may not always be identifiable.

In contrast, major aphthous ulceration typically manifests as painful, shallow ulcers rather than a persistent white patch. Oral candidiasis, also known as thrush, presents as creamy white patches that can be wiped away, differentiating it from leukoplakia. Lastly, squamous cell carcinoma may also present with white lesions, but these are more likely to have additional characteristics such as irregularity, bleeding, or ulceration, and are not described as simply an immovable patch without further signs or symptoms. Thus, leukoplakia stands out as the correct answer based on the specific features outlined in the question.