d
Particles will collide after time \(\mathrm{t}_{0}=\frac{\mathrm{h}}{\sqrt{2 \mathrm{gh}}}\)
at collision, \(\mathrm{v}_{\mathrm{A}}=\mathrm{gt}_{0} \quad \mathrm{v}_{\mathrm{B}}=\mathrm{u}_{\mathrm{B}}-\mathrm{gt}_{0}\)
\(\Rightarrow \mathrm{v}_{\mathrm{A}}=-\mathrm{v}_{\mathrm{B}}\)

Time taken by combined mass to reach the ground

time \(=\sqrt{\frac{2 \times 3 \mathrm{h} / 4}{\mathrm{g}}}=\sqrt{\frac{3 \mathrm{h}}{2 \mathrm{g}}}\)