Correct Answer - B
(b) The equivalent circuits are shown in the figure.
The circuit represents balanced Wheatstone Bridege.
Hence 6R Omega resistance is ineffective.
`1/(R_eq) = 1/(3R) + 1/(6R), R_(eq) = ((3R)(6R))/((3R) + (6R)) = 2R`
For Max. Power
External Resistance = Internal Resistance.
`2R = 4Omega :. R = 2Omega` .