Explanation:

The error is in fragment B. Subject here is ‘central banks’. The pronoun ‘they’ is altering the meaning of the sentence and making it absurd. Here ‘themselves’ should be used instead of ‘they’ in order to make the sentence correct. ‘Themselves’ is an empathic pronoun. It emphasizes the action done by the subject. The correct sentence would therefore be:

Besides, central banks themselves have been aggressive in buying physical gold to cushion falling currencies.

Hence, option B is the correct answer.