In the passage, I do not see where the author applies the rationale behind Shelley v. Kraemer to additional cases. Perhaps you are thinking of the end of the third paragraph. Here, the author demonstrates why the constitutional reasoning is flawed. State courts will enforce private contracts that violate the first amendment. Why should they have to respect the fourteenth amendment? This is meant to demonstrate the inconsistency in the Shelley v. Kraemer reasoning, not to extend it to other cases.
Even if you do consider this comparison to be "extending the rationale," this is only a brief segment of the passage and would not be considered the main point. Your decision to choose A was correct. The author spends the majority of the passage evaluating the reasoning behind this case. "Question" is the right word because the author is primarily critical of the rationale.
