"Blithe" describes someone cheerful, carefree and light-hearted, often in a way that suggests they don't worry much.
Its direct opposite is "Sad", meaning unhappy or sorrowful.
"Cheerful", "Ecstatic" and "Gladsome" are all words describing happiness in different degrees, so they're synonyms of blithe rather than its opposite, leaving Sad as the only genuine antonym among the four choices.