Correct Answer - D
(a) Initial flux through the coil, `phi_(bi) = +NBA`
Final flux through the coil, `phi_(bf) = -NBA`
When the coil is turned through `180^(@)` its flux reverses, the angle between magnetic field and areqa vector is reversed.
`Deltaphi_(B) = phi_(Bf) - phi_(Bi) = -NBA - (NBA) = -2NBA`
`Q = (Deltaphi_(B))/(R) = (2NBA)/(R)`