The correct option is B proportional to the square of their marked voltage
We know, P∝R and V∝R
Higher wattage means less resistance, when a number of bulbs are connected in series, a bulb with lower resistance will consume less power & the P.D. 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.