Answer is B.The RMS (Root mean square) value tells us what equivalent DC voltage we would need to get the same power, for the neon bulb to glow.In this case, the neon bulb glows at 50 V ac voltage. Therefore, for the bulb glow with dc voltage, 50\ imes \\sqrt { 2 } \\quad =\\quad 50\ imes 1.414\\quad =\\quad 70\\quad V dc voltage should be applied.Hence, the neon bulb will flash at 70 V dc.