Which of the following matrices is orthogonal (satisfies QTQ=IQ^T Q = IQTQ=I)?
(1101)\begin{pmatrix} 1 & 1 \\ 0 & 1 \end{pmatrix}(1011)
(1212−1212)\begin{pmatrix} \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\ -\frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \end{pmatrix}(21−212121)
(2001)\begin{pmatrix} 2 & 0 \\ 0 & 1 \end{pmatrix}(2001)
(10.50.51)\begin{pmatrix} 1 & 0.5 \\ 0.5 & 1 \end{pmatrix}(10.50.51)