When $10ml$ of $H_2$ and $12.5ml$ of ${Cl}_2$ are allowed to react, the final mixture contains under the same conditions

We know that 1mole substance contain 22.4L of it

Hence using unitary method , $10ml$ of $H_2$ contain 0.45 millimoles of it

Similarly, $12.5ml$ of ${Cl}_2$ contains 0.55 millimole

$H_2+{Cl}_2\to 2HCl$

According to the given reaction 1mole of each Hydrogen and Chlorine gas (i.e equal amounts) react to form Hydrogen chloride.

Thus here $H_2$ is limiting reagent.

Again acc. to reaction, 22.4L $H_2$ produce 44.8L $Hcl$

Here using unitary method $10ml$ of $H_2$ produce 20Ml $Hcl$

Now 0.1 millimole of ${Cl}_2$ remain unreacted which is around 2.5mL

Thus the mixture contains $20ml$ of $HCl$ and $2.5ml$ of ${Cl}_2$

Hence option (C) is the answer.