Correct option is (a) 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc)
The best I can explain: 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc).
With Bi determined by above equation, the ampere-turns needed to cancel the armature reaction ampere-turns and then to create the necessary flux density are given by, ATi = ATa (peak)+(Bi/µ0 )lgi