Which of the following matrices is an orthogonal matrix?
(1101)\begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix}(1011)
(1/2−1/21/21/2)\begin{pmatrix} 1/\sqrt{2} & -1/\sqrt{2} \\ 1/\sqrt{2} & 1/\sqrt{2} \end{pmatrix}(1/21/2−1/21/2)
(2002)\begin{pmatrix} 2 & 0 \\ 0 & 2 \end{pmatrix}(2002)
(1011)\begin{pmatrix} 1 & 0 \\ 1 & 1 \end{pmatrix}(1101)