Which word is an antonym of 'happy'?

The word 'sad' is an antonym of 'happy' because it conveys a state of emotional distress or unhappiness, which is directly opposite to the feeling of joy and contentment associated with happiness. In emotional vocabulary, antonyms reflect opposing feelings, and while 'joyful,' 'excited,' and 'cheerful' all represent positive emotions similar to happiness, 'sad' distinctly represents the absence of happiness and conveys a negative feeling. This contrast makes 'sad' the clear choice as an antonym.