The correct option is (C) 2R.
Explanation:
Let potential gradient along PQ is K then
VBA = Kℓ1 = K × 10 = 10K…….as (PJ = 10cm)
VBC = Kℓ2 = K × 30 = 30K…….as (PJ = 30cm)
∴ (VBA / VBC) = (10 / 30) = (1/3) and
(VBA / VBC) = [R / (R + X)]
∴ [R / (R + X)] = (1/3)
Hence 3R = R + X i.e. X = 2R