Which of the following matrices is nilpotent of index 2?
(1000)\begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix}(1000)
(0100)\begin{pmatrix} 0 & 1 \\ 0 & 0 \end{pmatrix}(0010)
(1101)\begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix}(1011)
(0110)\begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}(0110)