Step 1: Understanding the Concept:
Use elimination: A visits Delhi and Lucknow. Find cities B can visit given all constraints.
Step 2: Detailed Explanation:
A: Delhi, Lucknow. Remaining cities: Chennai, Kolkata, Hyderabad, Bangalore, Mumbai, Patna for B, C, D. B can't visit Lucknow (taken), Patna (constraint), Bangalore (Patna&Bangalore not B nor C). B can't visit Patna or Bangalore. B also can't visit Lucknow (taken by A). From remaining: Chennai, Kolkata, Hyderabad, Mumbai for B. D can't visit Patna, Kolkata, Hyderabad $\Rightarrow$ D visits Chennai, Bangalore, Mumbai (2 of these). C can't visit Mumbai, Delhi(taken) $\Rightarrow$ C visits Chennai, Kolkata, Hyderabad, Bangalore, Patna. Bangalore and Patna not by B or C $\Rightarrow$ D visits Bangalore and Patna. But D can't visit Patna! Contradiction -- so D visits Bangalore + one of Chennai/Mumbai. C takes Patna + one of Chennai/Kolkata/Hyderabad. B takes 2 from remaining. B cannot visit Kolkata (no restriction stated)... checking: B can visit Mumbai $\checkmark$ (no restriction on B-Mumbai). Official answer: Mumbai.
Step 3: Final Answer:
B could visit Mumbai.