The Mueller matrix for a quarter-wave plate with its fast axis at $+45^{\circ}$ is given by:
$$\left[\begin{array}{cccc}
1 & 0 & 0 & 0 \\
0 & 0 & 0 & -1 \\
0 & 0 & 1 & 0 \\
0 & 1 & 0 & 0
\end{array}\right]$$
which matches the given matrix. So, the given matrix is
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