C) 1.5 : 1
2011
$m{_1}$ = no of male students
$f{_1}$ = no of female students
$\dfrac{m{_1}}{f{_1}}=1$
2012
$m{_2}$ = no of male students
$f{_2}$ = no of female students
$\dfrac{m{_2}}{f{_2}}=1.5$
Given : $f{_1}=f{_2}=f$
So , $\dfrac{m{_1}}{f}=1$ and $\dfrac{m{_2}}{f}=1.5$
$f=\dfrac{m{_1}}{1}$ and $f=\dfrac{m{_2}}{1.5}$
Both the equations can be equated..
$\dfrac{m{_1}}{1} =\dfrac{m{_2}}{1.5}$
Ratio of male students in 2012 to male students in 2011 $\Rightarrow \dfrac{m{_2}}{m{_1}} =\dfrac{1.5}{1} = 1.5 : 1$