Step 1: Recall the four Brahmavihāras.
The four sublime states in Buddhism are Maitrī (loving-kindness), Karuṇā (compassion), Muditā (sympathetic joy), and Upekṣā (equanimity).
Step 2: Identify the relevant concept.
Muditā specifically refers to rejoicing in the happiness, success, and good fortune of others.
Step 3: Eliminate incorrect options.
(B) concerns goodwill toward all beings.
(C) relates to alleviating others’ suffering.
(D) denotes balanced detachment or equanimity.
Step 4: Conclusion.
Hence, the correct answer is Muditā.