b
\(\begin{array}{l}
mg\,\sin \theta  = ma\,\,\,\therefore \,a = g\sin \theta \\
where\,a\,is\,along\,the\,inclined\,plane\\
\therefore \,vertical\,component\,of\,acceleration\,\\
is\,g\,{\sin ^2}\theta \\
\therefore \,relative\,vertical\,acceleration\,of\,A\,with\,\\
\,respect\,to\,B\,is\\
g\,({\sin ^2}60 - {\sin ^2}30]\, = \frac{g}{2} = 4.9\,m/{s^2}\,in\\
vertical\,direction
\end{array}\)