Root 2 minus 1 is a unit so its ideal is not even proper

1. A.The ideal $2\mathbb{Z}[i]$ is maximal in $\mathbb{Z}[i]$.
2. B.The ideal $X\mathbb{C}[X,Y]$ is maximal in $\mathbb{C}[X,Y]$.
3. C.The set of all polynomials in $\mathbb{C}[X]$ whose coefficients add up to 0 is a maximal ideal in $\mathbb{C}[X]$.✓
4. D.The ideal $(\sqrt{2}-1)\mathbb{Z}[\sqrt{2}]$ is maximal in $\mathbb{Z}[\sqrt{2}]$.

The trap it tests

Hypothesis dropped

A theorem applied without checking one of its preconditions.