d
During vigorous exercise sufficient oxygen is not available to meet the required energy, so energy is drived through conversion of pyruvic acid to lactic acid.

\(\mathop {Glu\cos e}\limits_{Sloted\,in\,the\,from\,\,of\,Glycogen} \xrightarrow[\begin{subarray}{l} 
  (does\,not\,need\,oxy.\, \\ 
  need\,only\, \\ 
  \,enzymes) 
\end{subarray} ]{}\) \(\mathop {\mathop {Pyruvic\,acid\,}\limits_ \downarrow  }\limits_{Lactic\,acid} \xrightarrow{{{O_2}}}C{O_2} + {H_2}O\)