C is the only possible correct answer. This is tricky because it is a long phrase that is being substituted and it is difficult to keep the entire sentence in one's head. If we shorten it a bit, it is clearer, I think. (C) becomes then "The Supreme Court declared as unconstitutional a minimum wage, ruling that it was a form of price-fixing. The other options are either incomplete sentences (E) or only work if we change "ruling" to "ruled" (A and B) or require us to rearrange the sentence (D). This question takes some very careful reading!

Hi. I’m a native English speaker and a lawyer. While each of the statements is reasonable, each can be improved. If you must choose from these, “D” is best. One problem is the word “declared.” Better choices would be “struck down” or “ruled,” because the Supreme Court doesn’t really make declarations. It decides if a law is constitutional or not. A smaller problem is that the phrase “minimum wage” is incomplete here. A better choice might be “minimum wage law”. A minimum wage is a concept; the Supreme Court didn’t reject the concept, per se. It rejected a DC law establishing a minimum wage for women (or more precisely, establishing a board to oversee minimum allowed wages). Consider his alternative: “In 1923, the Supreme Court struck down a District of Columbia law mandating minimum wage requirements for women. The court ruled that the law unconstitutionally infringed upon the rights of employers and employees to contract on their own terms, as protected by the Fifth Amendment.” I hope this helps.

A, B, C, and D are all correct. What a strange question! Actually, all of them are technically wrong. It was the law establishing the minimum wage that was declared unconstitutional, if you want to get picky. The Supreme Court cannot issue proclamations about what should or should not be. That would be a law. Only Congress can make laws.