Solution
= Solution
Write $x=x_1x_2$. Since the marked word is $11$, the required oracle is a single <Toffoli gate> with $x_1$ and $x_2$ as its two controls and the answer qubit $y$ as its target:
$$
|x_1x_2\rangle|y\rangle
\longmapsto
|x_1x_2\rangle|y\mathbin\oplus(x_1x_2)\rangle.
$$
The target flips exactly when both controls are one, which is exactly when $x=11$.