Step-by-step Solution:

The word "debacle" means a sudden and ignominious failure; a fiasco. We need to find the most nearly similar synonym from the options.

• (A) Dandy: Refers to a man concerned with style or something excellent, not related to failure.
• (B) Corker: Means excellent or astonishing, which is opposite to debacle.
• (C) Catastrophe: Means a disaster or sudden damage, which is very similar to debacle.
• (D) Opulence: Means great wealth, unrelated to failure.

Therefore, the correct synonym is (C) Catastrophe.