Chọn phương án D.

Ta có \(\overrightarrow{OM}=(-2;-1)\), \(\overrightarrow{ON}=(3;-1)\).

Khi đó:
\(\begin{eqnarray*}\cos\widehat{MON}&=&\cos\left(\overrightarrow{OM},\overrightarrow{ON}\right)\\
&=&\dfrac{-2\cdot3-1\cdot(-1)}{\sqrt{(-2)^2+(-1)^2}\cdot\sqrt{3^2+(-1)^2}}\\
&=&-\dfrac{\sqrt{2}}{2}.\end{eqnarray*}\)

Suy ra \(\widehat{MON}=135^\circ\).