b
(b)When ball falls vertically downward from height \({h_1}\) its velocity \({\overrightarrow v _1} = \sqrt {2g{h_1}} \)
and its velocity after collision \({\overrightarrow v _2} = \sqrt {2g{h_2}} \)
Change in momentum
\(\Delta \vec P = m({\overrightarrow v _2} - {\overrightarrow v _1}) = m(\sqrt {2g{h_1}} + \sqrt {2g{h_2}} )\)
(because \({\overrightarrow v _1}\) and \({\overrightarrow v _2}\) are opposite in direction)