Đáp án:

•   \(\cos (\overrightarrow{a}, \overrightarrow{b}) = \frac{\overrightarrow{a}. \overrightarrow{b}}{|\overrightarrow{a}| |\overrightarrow{b}|} = \frac{-2}{\sqrt{5} . \sqrt{5}} = -\frac{2}{5} \quad\)

\(\Rightarrow\) Vậy chọn \(\boxed{\text{B}} \)