If $\vec{a}, \vec{b}$ and $\vec{c}$ are three vectors such that $\vec{a}+\vec{b}+\vec{c}=\vec{0}$, where $\vec{a}$ and $\vec{b}$ are unit vectors and $|\vec{c}|=2$, then the angle between the vectors $\vec{b}$ and $\vec{c}$ is:

180°

The correct answer is Option (4) → 180°

$\vec{a}=-(\vec{b}+\vec{c})$

squaring both sides

$|\vec a|^2=|\vec b|^2+|\vec c|^2+2\vec{b}.\vec{c}$

$1=1+4+2×1×2\cos θ$

$\cos θ=-1$

$θ=180°$