The heat of combustion of benzene at $27°C$  and at constant volume is -3245 KJ. Then the heat of combustion of benzene at the same temperature and at constant pressure is

The reaction is combustion of benzene is given below:

${\mathrm{C}}_6{\mathrm{H}}_6+\frac{15}{2}{\mathrm{O}}_2\to 6{\mathrm{CO}}_2+3{\mathrm{H}}_2\mathrm{O}$

At constant pressure, $q=\Delta H$ 

${\mathrm{\Delta n}}_{\mathrm{g}}=9-7.5=1.5$

We can use the formula $\mathrm{\Delta H}=\mathrm{\Delta U}+\left({\mathrm{\Delta n}}_{\mathrm{g}}\right)\mathrm{RT}$.

Putting the values, we get:

$$\begin{gathered} \mathrm{\Delta H}=-3245+\frac{1.5\times 8.314\times 300}{1000} \\ \mathrm{\Delta H}=-3241.26\text{ KJ} \end{gathered}$$

Therefore, the correct option is A.