Which of the following matrices is an identity matrix?
(1111)\begin{pmatrix} 1 & 1 \\ 1 & 1 \end{pmatrix}(1111)
(0110)\begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}(0110)
(1001)\begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}(1001)
(1000)\begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix}(1000)