Past exam of the mathematics course of the University of Cambridge / 2025 / ii / Paper 3 / 10C / b / iv / Solution

Only one Grover iteration is required. For one marked item among four, the initial marked amplitude corresponds to an angle $\theta$ with $\sin \theta =1/2$, so $\theta =\pi /6$; one iteration changes this to $3\theta =\pi /2$, giving unit success amplitude. Further iterations are neither needed nor beneficial.