Which among the following is a unitless physical quantity?

Two solutions labeled as $3\,M\,HCl$ and $1\,M\,HCl$ are mixed in the ratio of $x : y$ by volume and the molarity of the resulting solution is $1.5\,M$. The molarity of the resulting solution if they are mixed in the ratio $y : x$ by volume is ....... $M$.

$63 \ g$ of a compound (Mol. Wt. $= 126$) was dissolved in $500 \ g$ of distilled water. The density of the resultant solution is $1.126 \ g / mL$. The molarity of the solution is: (in $M$)

$100 \ mL$ of $\frac{M}{10} \ Ca(NO_3)_2$ and $200 \ mL$ of $\frac{M}{10} \ KNO_3$ solutions are mixed. What is the normality of the resulting solution with respect to $NO_3^{-}$ ions (in $N$)?

$A$ solution of a solute in benzene has a molality of $X \ m$. If the mole fraction of the solute is $0.2$,what is the value of $X$?

What is the final molarity of a solution obtained by mixing $480 \, mL$ of $1.5 \, M$ solution and $520 \, mL$ of $1.2 \, M$ solution (in $, M$)?