We should select the verb with correct tense form considering the context. Here, we have the word 'next month', it means it would take place in future. Hence, we should use the future tense.
Option A tells that Sunder would have been in the process of leaving for America next month, i.e. in the future perfect continuous tense. So, it is correct.
Option B tells that Sunder would be in the on-going stage of leaving for America. So, it is correct.
Option C tells that Sunder would have completed the action of leaving for America. So, it is correct.
Option D isn't in the right sentence construction. So, it is not correct.