Step 1: Jurisdiction rules.
In contract cases, a suit can be filed where the cause of action arose or where the defendants reside.
Step 2: Application.
- Cause of action arose at Banaras.
- B resides at Kolkata.
- C resides at Delhi.
Step 3: Conclusion.
Thus, A may sue at Banaras, Kolkata, or Delhi. Hence answer is (D).