Step 1: The test for ideal behaviour.
A solution counts as ideal when the pressure we actually measure lands exactly on the value Raoult's law predicts. If the real pressure comes out higher than predicted it is positive deviation, lower means negative deviation. So we just predict and compare.
Step 2: Mole fractions of the two liquids.
Total moles are $1 + 2 = 3$, so
\[ x_A = \tfrac{1}{3}, \qquad x_B = \tfrac{2}{3} \]
Step 3: Predict the pressure.
\[ P_{pred} = x_A P_A^\circ + x_B P_B^\circ = \tfrac{1}{3}(30) + \tfrac{2}{3}(45) = 10 + 30 = 40\ \text{torr} \]
Step 4: Compare and decide.
The measured total pressure is also 40 torr, identical to the prediction. With zero gap between them there is no deviation either way, so the solution behaves ideally.
\[ \boxed{\text{Option (A): an ideal solution}} \]