The standard form of  $\frac{36}{108}$ is

Standard form of a rational number ( simplest form of a rational number ):

A rational number $\frac{p}{q}$ is said to be in standard form, if  $p$ and $q$ are integers having no common divisor other than 1 , where $q$ is positive $(q\ne 0)$.

Option ( C ) : $\frac{36}{108}=\frac{1\times 36}{3\times 36}=\frac{1}{3}$

Option ( A ) : $\frac{12}{24}$

$\frac{12}{24}=\frac{1\times 12}{2\times 12}=\frac{1}{2}$             $\left(\because \frac{{\displaystyle 1}}{{\displaystyle 2}}\ne \frac{{\displaystyle 1}}{{\displaystyle 3}}\right)$

 $\therefore \frac{12}{24}$ it is not the standard form of $\frac{36}{108}$

Option ( B ) : $\frac{5}{12}$ 

$\frac{5}{12}=\frac{1\times 5}{3\times 4}$      ( when they have no common divisor )

 $\therefore \frac{5}{12}$ it is not the standard form of $\frac{36}{108}$ 

Option ( D ) : $\frac{6}{12}$ 

$\frac{6}{12}=\frac{1\times 6}{2\times 6}=\frac{1}{2}$              $\left(\because \frac{{\displaystyle 1}}{{\displaystyle 2}}\ne \frac{{\displaystyle 1}}{{\displaystyle 3}}\right)$ 

  $\therefore \frac{6}{12}$ it is not the standard form of $\frac{36}{108}$