This question evaluates the understanding of idiomatic expressions. The correct answer is D, as 'a wolf in sheep's clothing' is a well-known idiom that describes someone who hides malicious intent under a friendly facade. Option A misuses 'heart of stone,' which typically refers to someone who is emotionally unresponsive, but is not a fixed expression. Option B uses 'kicked the bucket' correctly, but it is less formal. Option C uses 'in hot water' correctly, but it is not as universally recognized as D.