Chọn C
Đặt: \(z=a+bi\, {\rm \; }\left(a,b\in {\rm R}\right).\)
Ta có \(3\left(a-bi+i\right)-\left(2-i\right)\left(a+bi\right)=3+10i\)
\(\Leftrightarrow 3a-3bi+3i-2a-2bi+ai+bi^{2} -3-10i=0\)
\(\Leftrightarrow \left(a-b-3\right)+\left(a-5b-7\right)i=0 \)
\(\Leftrightarrow \left\{\begin{array}{l} {a-b-3=0} \\ {a-5b-7=0} \end{array}\right. \Leftrightarrow \left\{\begin{array}{l} {a=2} \\ {b=-1} \end{array}\right. \)
Do đó: \(\left|z\right|=\sqrt{2^{2} +\left(-1\right)^{2} } =\sqrt{5} \)