Statement (A) : At constant temperature graphs drawn are called Isobars 

Statement (B) : At constant temperature the volume of given mass of gas is directly proportional to pressure of gas. 

Statement (C) : According to Boyle’s Law: ${\mathrm{P}}_1{\mathrm{V}}_1={\mathrm{P}}_2{\mathrm{V}}_2$

The graphs obtain for gases at constant temperature are called isotherm. Isobars means the graphs obtain at constant pressure of gas.

From Boyle’s Law $\Rightarrow \text{P}\propto \frac{\text{1}}{\text{V}}$

                   $\text{P}=\frac{\text{K}}{\text{V}}\Rightarrow \text{PV}=\text{K}$

If a fixed amount of gas at constant temperature T, occupying volume ${\text{V}}_{\text{1}}$ at pressure${\text{P}}_1$ , undergoes expansion so that volume becomes ${\text{V}}_{\text{2}}$  and pressure becomes${\text{P}}_2$ . According to Boyle’s Law

$\frac{{\text{P}}_1}{{\text{P}}_2}=\frac{{\text{V}}_2}{{\text{V}}_1}\Rightarrow {\text{P}}_1{\text{V}}_1={\text{P}}_2{\text{V}}_2=\text{constant}$

Therefore, statements I and II are incorrect but statement III is correct.