The correct answer is: C. obligatory, submit.
The word “obligatory” means “required or expected to be done.” The word “submit” means “to send something to a person or organization in order to be dealt with.” In this case, the tax returns are being sent to the Income Tax Department in order to be processed.
The other options are not correct because they do not accurately reflect the meaning of the sentence. The word “necessary” does not convey the sense of obligation that is implied in the sentence. The word “binding” is too strong, as it suggests that there is a legal requirement to submit tax returns. The word “possible” does not accurately reflect the fact that submitting tax returns is a requirement, not an option. The word “worthwhile” does not accurately reflect the fact that submitting tax returns is a legal obligation, not a matter of personal choice.
In conclusion, the correct answer is: C. obligatory, submit.