In Choose odd pair of words type of questions, certain pairs of words are given out of which the words in all the pairs except one, bear a certain common relationship. The candidate is required to decipher this relationship and chose the pair in which the words are differently related, as the answer.
(China : Beijing), (Russia : Moscow), (Japan : Singapore), (Spain : Madrid)
Clearly, the answer is (c).
In all other pairs, second is the capital of first.