A neon lamp is connected to a voltage a.c. source. The voltage is gradually increased from zero volt. It is observed that the neon flashes at $50V$. The a.c, source is now replaced by a variable dc source and the experiment is repeated. The neon bulb will flash at ?

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, $50\times \sqrt{2}=50\times 1.414=70V$ dc voltage should be applied.
Hence, the neon bulb will flash at 70 V dc.