d
\(cos\theta \,\, = \,\,\frac{{\mathop {\rm{A}}\limits^ \to \,\mathop {\rm{B}}\limits^ \to }}{{|\mathop {\rm{A}}\limits^ \to |\,\,|\mathop {\rm{B}}\limits^ \to |}}\,\, = \,\,\frac{{ - 6\,\, + \;\,6}}{{\sqrt {14} \,\,\sqrt {45} }}\,\, = \,\,0\,\,\,\, \Rightarrow \,\,\theta \,\, = \,\,90^\circ \)