d
\(\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,Initial\,velocity\,u = 20m/s;\\
m = 1\,kg\,Kinetic\,energy\, = {\rm{maximum}}\\
potential\,energy\,\\
Initial\,kinetic\,energy\, = \frac{1}{2} \times 1 \times {20^2}\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 200\,J\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Mgh\,(\max ) = 200\,J.
\end{array}\)

\(\begin{array}{l}
\therefore \,\,\,h = 20\,m.\\
The\,height\,travelled\,by\,the\,body,\\
h' = 18\,m\\
\therefore \,Loss\,of\,energy\,due\,to\,air\,friction\\
 = mgh - mgh'\\
 \Rightarrow \,Energy\,lost\, = 200\,J - 1 \times 10 \times 18\,J\,\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 20\,J.
\end{array}\)