Acetone $(\mathrm{BP} 329\mathrm{K})$ and ${\mathrm{CS}}_2 (\mathrm{BP} 320 \mathrm{K})$ are mixed in a definite composition so that the mixture of the two behave like a pure liquid and boils at $312 \mathrm{K}$, then it is

Option(b) is correct.

We know that the solutions which show large positive deviations from Raoult's law form maximum boiling azeotropes.

The boiling point of $\mathrm{Acetone}$$=329 \mathrm{K}$

The Boiling point of ${\mathrm{CS}}_2= 320 \mathrm{K}$

Boiling point of solution$=312\mathrm{K}$(given)

${\mathrm{Bp}}_{\mathrm{Acetone}} ,{\mathrm{Bp}}_{{\mathrm{CS}}_2}> {\mathrm{Bp}}_{\mathrm{soln}}$

Therefore, $\mathrm{Acetone}$ and ${\mathrm{CS}}_2$ forms maximum boiling azeotropes.