let $x$ is the number of boys, $y$ is the number of girls.
average score of boys $(A)=71$
average score of girls $(B)=73$
The average score of the school= $\frac{A*x+B*y}{x+y}$
$\implies 71.8=\frac{71*x+73*y}{x+y}$
$\implies71.8x+71.8y=71x+73y$
$\implies x(71.8-71)=y(73-71.8)$
$\implies 0.8x=1.2y$
$\implies x:y=3:2$
Option $(C)$ is correct.