a
\(\begin{array}{l}
For\,downward\,motion\,v =  - gt\\
The\,velocity\,of\,the\,rubber\,ball\,increases\,in\,dowanward\,\\
direction\,and\,we\,get\,a\,straight\,line\,between\,v\,and\,t\,with\,\\
a\,nagative\,slope.\\
Also\,applying\,y - {y_0} = ut + \frac{1}{2}a{t^2}\\
We\,get\,y - h =  - \frac{1}{2}g{t^2} \Rightarrow y = h - \frac{1}{2}g{t^2}\\
The\,graph\,between\,y\,and\,t\,is\,a\,parabola\,with\,y = h\,at\\
t = 0.\,As\,time\,increases\,y\,decreases.\\
For\,upward\,motion.\\
The\,ball\,suffer\,elastic\,collision\,with\,the\,horizonatl\,elastic\\
plate\,therefore\,the\,direction\,of\,velocity\,is\,reversed\,and\,\\
the\,magunitude\,remains\,the\,same.\\
Here\,v = u - gt\,where\,u\,is\,the\,velocity\,just\,after\,collision.\\
As\,t\,increases,\,v\,decreases.\,We\,get\,a\,straight\,line\,between\\
\,v\,and\,t\,with\,negative\,slope.\\
Also\,\,y = ut - \frac{1}{2}g{t^2}\,
\end{array}\)