a
\(\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,\,\,\,\,When\,the\,mass\,attached\,to\,a\,spring\\
fixed\,at\,the\,other\,end\,is\,allowed\,to\,fall\\
suddenly,\,it\,extends\,the\,spring\,by\,x.\,\\
potential\,energy\,lost\,by\,the\,mass\,is\\
gained\,by\,the\,spring.\\
Mgx = \frac{1}{2}k{x^2} \Rightarrow x = \frac{{2Mg}}{k}.
\end{array}\)