An open vessel at 27°C is heated until 3/4 mass of the air in it has been expelled. Assuming that the volume of the vessel remains constant, the temperature to which the vessel has been heated is 

Open vessel Temperature=27+273=300 K

Mass =¾ times expelled 

moles of gas at 273 K be n=${\mathrm{n}}_1$

​∴ moles of air left at T=$\mathrm{n}-\frac{3}{4}\mathrm{n}=\frac{\mathrm{n}}{4}$

Same condition of pressure and volume then

${\mathrm{n}}_1{\mathrm{T}}_1={\mathrm{n}}_2{\mathrm{T}}_2$

$\begin{array}{c}\mathrm{n}\times 300=\frac{\mathrm{n}}{4}\times \mathrm{T}\end{array}$

T=1200 K

Therefore, the required option is B