b
\(\begin{array}{l}
Velocity\,at\,highest\,{\rm{point}}\,becomes\,zero\\
\therefore \,0 = u - at\\
or\,u = at\\
\,\,\,\,\,\,\,\, = \,7.5 \times 3.5 = 62.25m/s\\
\,\,\,\,\,\,\,\,\,{y_1} = u \times 1 - \frac{1}{2} \times 7.5 \times {1^2}\\
\,\,\,\,\,\,\,\,\,{y_2} = u \times 2 - \frac{1}{2} \times 7.5 \times {2^2}\\
\,\,\,\,\,\,\,\,\,h = {y_2} - {y_1} = 15
\end{array}\)