The question assesses understanding of synonyms, which are words with similar meanings. * **A is incorrect:** 'Hot' and 'cold' are antonyms, with opposite meanings. * **B is incorrect:** 'Fast' and 'slow' are also antonyms, not synonyms. * **C is correct:** 'Happy' and 'joyful' share similar meanings, making them synonyms. * **D is incorrect:** 'Up' and 'down' are antonyms, not synonyms. The core idea is to recognize words with nearly identical meanings are synonyms. **Core Concept:** Synonyms, Word Relationships **Solution Strategy:** Identify words with similar meanings. **Option A Analysis:** 'Hot' and 'cold' have opposite meanings, not similar. **Option B Analysis:** 'Fast' and 'slow' have opposite meanings, not similar. **Option C Analysis:** 'Happy' and 'joyful' have similar meanings, so they are synonyms. **Option D Analysis:** 'Up' and 'down' have opposite meanings, not similar. **Common Mistake:** Confusing synonyms with antonyms or unrelated words.