d
\(A\xrightarrow[(ii)\;H_3O^+]{(i)\;CH_3MgBr}B\xrightarrow[573\;K]{Cu}2-\)methyl \(2-\)butene

\(\begin{array}{*{20}{c}}
  {C{H_3} - C - C{H_2} - C{H_3}} \\ 
  {||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} 
\end{array}\xrightarrow[{{H_3}{O^ + }}]{{C{H_3}MgBr}}\)\(\begin{array}{*{20}{c}}
  {C{H_3}\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {C{H_3} - C - C{H_2} - C{H_3}} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {OH\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} 
\end{array}\xrightarrow[{573\,K}]{{Cu}}\)\(\begin{array}{*{20}{c}}
  {C{H_3}\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {C{H_3} - C = CH - C{H_3}} 
\end{array}\)

\(\mathrm{C} \Rightarrow 12 \times 4=48\)

\(\mathrm{H} \Rightarrow 8 \times 1=8\)

\(\mathrm{O} \Rightarrow 16 \times 1=16\)

Total \(= 72\)

\(\%\) of \(\mathrm{C}=\frac{48}{72} \times 100=66.66 \%\)