The question is testing the understanding of synonyms in English vocabulary. The correct answer is 'B. Joyful' because it is a word that has a similar meaning to 'happy'. Option A is incorrect because 'Sad' is an antonym of 'happy'. Option C is incorrect because 'Angry' is another emotion that is not synonymous with 'happy'. Option D is incorrect because 'Tired' refers to a state of fatigue, not a feeling of happiness. The core concept here is the recognition of synonym pairs.