Chọn phương án C.

\(\begin{aligned}
&\begin{cases}
u_1+u_7=26\\
u_2^2+u_6^2=466
\end{cases}\\
\Leftrightarrow&\begin{cases}
u_1+\left(u_1+6d\right)=26\\
\left(u_1+d\right)^2+\left(u_1+5d\right)^2=466
\end{cases}\\
\Leftrightarrow&\begin{cases}
2u_1+6d=26\\
\left(u_1+d\right)^2+\left(u_1+5d\right)^2=466
\end{cases}\\
\Leftrightarrow&\begin{cases}
u_1=13-3d\\
\left(u_1+d\right)^2+\left(u_1+5d\right)^2=466
\end{cases}\\
\Leftrightarrow&\begin{cases}
u_1=13-3d\\
\left(13-2d\right)^2+\left(13+2d\right)^2=466
\end{cases}\\
\Leftrightarrow&\begin{cases}
u_1=13-3d\\
8d^2-128=0
\end{cases}\\
\Leftrightarrow&\begin{cases}
u_1=13-3d\\
\left[\begin{array}{l}d=4\\ d=-4.\end{array}\right.
\end{cases}\\
\Leftrightarrow&\left[\begin{array}{l}d=4,\,u_1=1\\ d=-4,\,u_1=25.\end{array}\right.
\end{aligned}\)