DIVIDE and UNITE are opposites—one means to break apart, the other to bring together.
Similarly, Marriage (a union) and Divorce (a separation) are perfect antonyms.
Other options:
(a) Split and Apart are synonyms, not opposites.
(b) Fission and Fusion are opposites too, but more technical in nature, not as directly parallel.
(d) Chasm and Gap are not opposites—they are near synonyms.
Hence, (c) is the clearest and most relatable antonymic pair.