From a high tower, at time t = 0, one stone is dropped from rest and simultaneously another stone is projected vertically up with an initial velocity. The graph of distance
S between the two stones plotted against time t will be  

At time t, let the displacement of first stone be ${\mathrm{S}}_1=\frac{1}{2}{\mathrm{gt}}^2$ and that of the second stone be

${\mathrm{S}}_2=\mathrm{ut}-\frac{1}{2}{\mathrm{gt}}^2$

Distance between two stones at time t:

$\mathrm{S}={\mathrm{S}}_1+{\mathrm{S}}_2=\mathrm{u}\Rightarrow \mathrm{S}=\mathrm{ut}$

So the graph should be a straight line passing through origin as shown in option (a).