c
$P=\left[\begin{array}{cc}\frac{\sqrt{3}}{2} & \frac{1}{2} \\ -\frac{1}{2} & \frac{\sqrt{3}}{2}\end{array}\right]$
$P^{T}=\left[\begin{array}{cc}\frac{\sqrt{3}}{2} & \frac{-1}{2} \\ \frac{1}{2} & \frac{\sqrt{3}}{2}\end{array}\right]$
$P P^{T}=P^{T} P=I$
$\mathrm{Q}^{2015}=\left(P A P^{T}\right)\left(P A P^{T}\right)-(2015 \text { terms })$
$=P A^{2015} P^{T}$
$P^{T} \mathrm{Q}^{2015} P=A^{2015}$
$A^{2}=\left[\begin{array}{ll}1 & 1 \\ 0 & 1\end{array}\right]\left[\begin{array}{ll}1 & 1 \\ 0 & 1\end{array}\right]=\left[\begin{array}{ll}1 & 2 \\ 0 & 1\end{array}\right]$
$A^{3}=\left[\begin{array}{ll}1 & 2 \\ 0 & 1\end{array}\right]\left[\begin{array}{ll}1 & 1 \\ 0 & 1\end{array}\right]=\left[\begin{array}{ll}1 & 3 \\ 0 & 1\end{array}\right]$
$A^{2015}=\left[\begin{array}{cc}1 & 2015 \\ 0 & 1\end{array}\right]$