Chọn phương án A.

Ta có:

• \(\vec{a}\cdot\vec{b}=4\cdot(-1)+3\cdot(-7)=-25\)
• \(\left|\vec{a}\right|=\sqrt{4^2+3^2}=5\)
• \(\left|\vec{b}\right|=\sqrt{(-1)^2+(-7)^2}=5\sqrt{2}\)

Khi đó: $$\begin{eqnarray*}\cos\left(\vec{a},\vec{b}\right)=\dfrac{\vec{a}\cdot\vec{b}}{\left|\vec{a}\right|\cdot\left|\vec{b}\right|}&=&\dfrac{-25}{5\cdot5\sqrt{2}}\\ &=&-\dfrac{\sqrt{2}}{2}.\end{eqnarray*}$$Suy ra \(\left(\vec{a},\vec{b}\right)=135^\circ\).