Chọn phương án C.

\(\begin{aligned}
&\begin{cases}
u_7-u_3=8\\
u_2\cdot u_7=75
\end{cases}\\
\Leftrightarrow&\begin{cases}
\left(u_1+6d\right)-\left(u_1+2d\right)=8\\
\left(u_1+d\right)\left(u_1+6d\right)=75
\end{cases}\\
\Leftrightarrow&\begin{cases}
4d=8\\
\left(u_1+d\right)\left(u_1+6d\right)=75
\end{cases}\\
\Leftrightarrow&\begin{cases}
d=2\\
\left(u_1+2\right)\left(u_1+12\right)=75
\end{cases}\\
\Leftrightarrow&\begin{cases}
d=2\\
u_1^2+14u_1-51=0
\end{cases}\\
\Leftrightarrow&\begin{cases}
d=2\\
\left[\begin{array}{l}u_1=3\\ u_1=-17.\end{array}\right.
\end{cases}
\end{aligned}\)