The circuit can be recognized as a balanced Wheatstone bridge. The central galvanometer arm can be ignored. The upper branch has resistance 15+10=25Ω. The lower branch has 30+20=50Ω. The left vertical resistors are not part of the main path between A&B. This seems to be a complex bridge that simplifies differently. (Explanation: The circuit is a balanced bridge with ratio 10/20 = 15/30. So the middle wire has no current. Req = (10+15)||(20+30) = 25||50 = 50/3 Ω. This doesn't match the options. The key is likely based on a different interpretation.)