① In line with the meaning of the question, RMB's participation in SDR currency basket is an important milestone in RMB internationalization, which shows that RMB's international credit is constantly strengthening;

② In line with the meaning of the question, RMB joining SDR currency basket will reduce the trouble and risk of residents' exchange rate exchange, thus reducing the exchange cost of China residents going abroad;

③ Nothing, RMB's participation in SDR currency basket has nothing to do with China's foreign exchange reserve risk;

④ It doesn't matter. The RMB exchange rate is influenced by many factors. Joining the SDR currency basket may not keep the RMB exchange rate stable.

Therefore, choose a in this question.