Gases.....................

It is a fact that $\log{P} =\log{nRT} -\log{V}$ for an ideal gas; this is a consequence of the ideal gas law. Assuming the author meant that it is an isothermal process, $\log{P} = C - \log{V}$ where C is a constant. This is a line of slope $m = -1$ which means the angle with respect to the $V$ axis is $\arctan{(-1)} = -45^{\circ}$ Now the answer is a matter of semantics and poor definition. Since $45^{\circ}$ isn't an answer, perhaps it is the supplementary angle which constructs the same slope: $180^{\circ} - 45^{\circ} = 135^{\circ}$ which is an answer.