Here's the official explanation provided by the GMAC for this question:

The underlined portion of this sentence provides the subject for the sentence's main verb violates. This subject should be expressed clearly and concisely. The adverbial comparative as prominently as must be used idiomatically to express the intended comparison between how prominently the highest price is advertised and how prominently the lowest price is advertised.

Option A: The construction if you fail …, it is unnecessarily wordy and inappropriate in formal writing. The antecedent of it is unclear. The antecedent of that in that of the lowest is also unclear--the sentence is meant to discuss failure to advertise the highest price as prominently as the lowest price, not failure to advertise the highest price as prominently as that of the lowest price.

Option B: The construction if one fails …, it is unnecessarily wordy, and the antecedent of it is unclear.

Option C: Correct. The noun phrase starting with failure provides a clear subject for violates. The comparative as prominently as is used appropriately to compare how prominently the highest price is advertised with how prominently the lowest price is advertised.

Option D: Inserting the long phrase the highest price in a range of prices for a service or product in the middle of the comparative as prominently … as makes the sentence unnecessarily confusing and difficult to parse.

Option E: Inserting the highest price in a range of prices for a service or product in the middle of as prominently … as makes the sentence unnecessarily confusing and difficult to parse. The antecedent of that in that of the lowest is unclear.

The correct answer is C.

Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.
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