$f(x) =ce^{-\Large\frac{1}{2} \bigg(\frac{(x-\mu)^2}{\sigma}\bigg)}$
Comparing with $f(x) =\large\frac{1}{\sigma \sqrt {2 \pi}}e^{-\Large\frac{1}{2} \bigg(\frac{(x-\mu)^2}{\sigma}\bigg)}$
$\mu=100, \sigma =5$
$e= \large\frac{1}{\sigma \sqrt {2 \pi}}=\frac{1}{5 \sqrt {2 \pi}}$
Hence 4 is the correct answer.