d
\(\begin{array}{l}
\,{\rm{|}}\mathop {\rm{A}}\limits^ \to + \;\mathop B\limits^ \to \,|= {\rm{|}}\mathop {\rm{A}}\limits^ \to \,\, - \;\,\mathop B\limits^ \to \,|\,\, \\ \Rightarrow \,\,\sqrt {{A^2} + \;{B^2}+ \;2AB \cos \theta } = \sqrt {{A^2} + \;\,{B^2}\, - 2AB \cos \,\theta } \\
\Rightarrow \,\,{A^2} + \;{B^2}+ \;\;2AB\,\,\cos \,\,\theta \,\, = \,\,{A^2} + {B^2} - 2AB\cos \theta \\ \Rightarrow cos \theta = 0\, \\ \therefore \,\theta = 90^\circ
\end{array}\)