 Solution

Let $a,b$ be as in i. Insert the explicit oracle from ii between the two pairs of Hadamard gates. The network identitiesturn the entire network intoThis is the last simplified circuit in i. On the designated input $|0{⟩}_x|1{⟩}_y$, its reversed control is certainly one, soThe global phase is unobservable. Measuring the first wire therefore determines the function class with certainty after one oracle query, while the second wire returns to $|1⟩$. These circuit identities are the network form of quantum phase kickback: the answer state between the Hadamards is the $X$ eigenstate $|-⟩$, on which an XOR acts as a phase.