We're dealing with a gas here, so keeping the Ideal Gas Law

pV = nRT

in mind, is a good idea

We're not interested in the type of gas, i.e. moles (so we don't need R either)
and the pressure remains constant, pV = nRT so we'll end up with

V = T, also called Charles's Law

Since this is the same gas under different conditions,
we want to set up a volume to temperature ratio

Celcius

converting cm3 to L 268 cm3 1 mL / 1 cm3 1 L / 1000 mL = 0.268 L 648 cm3 1 mL / 1 cm3 1 L / 1000 mL = 0.648 L converting °C to K 18°C + 273.15 = 291.15 K

Charles's Law Example

Vinitial / Tinitial = Vfinal / Tfinal plugging in the provided info yields 0.268 L / 291.15 K = 0.648 L / T T = 703.97 K

Kelvin

703.97 K - 273.15 = 430.82 °C

Option A is the correct answer