Correct option C
Explanation:
\(P=\frac{V^2}R=\frac{V'^2}{R'}\)
Higher watt means less resistance, when a number of bulbs are connected in series, a bulb with lower resistance will consume less power & the P.D (potential difference) across it will also be less. Therefore in series connection, a bulb with higher voltage will glow less brightly. Hence, their brightness will be proportional to the square of their marked voltage.