The transformation matrix for mirroring a point in $x – y$ plane about the line $y=x$ is given by

1. $\begin{bmatrix} 1 & 0 \\ 0 & -1 \end{bmatrix} \\$
2. $\begin{bmatrix} -1 & 0 \\ 0 & 1 \end{bmatrix} \\$
3. $\begin{bmatrix} 0 & 1 \\ 1 & 0 \end{bmatrix} \\$
4. $\begin{bmatrix} 0 & -1 \\ -1 & 0 \end{bmatrix}$