This time tomorrow, we will sitting in the train on our way to Patna. 

Explanation-Here will sitting is wrongly used. As we know will and shall represent the probable action for the future time and they are also modals. So, with modals we use base form of the verb so we cannot use ‘sitting’ with will. In future continuous tense we use Will/shall + be + v1+ing. Therefore, Will be sitting is correct. So option (B) is correct.