Which of these matrices represents a reflection across the line y=xy=xy=x?
(0110)\begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}(0110)
(100−1)\begin{pmatrix} 1 & 0 \\ 0 & -1 \end{pmatrix}(100−1)
(−1001)\begin{pmatrix} -1 & 0 \\ 0 & 1 \end{pmatrix}(−1001)
(0−1−10)\begin{pmatrix} 0 & -1 \\ -1 & 0 \end{pmatrix}(0−1−10)