The teacher instructed three students $A$,$B$ and $C$ respectively to prepare a $50\%$ (mass by volume) solution of sodium hydroxide $(NaOH)$. $A$ dissolved $50\,g$ of $NaOH$ in $100\,mL$ of water,$B$ dissolved $50\,g$ of $NaOH$ in $100\,g$ of water while $C$ dissolved $50\,g$ of $NaOH$ in water to make $100\,mL$ of solution. Which one of them has made the desired solution and why?

(C) Student $C$ has made the desired solution.
The formula for mass by volume percentage is:
$\text{Mass by volume } \% = \frac{\text{Mass of solute (g)}}{\text{Volume of solution (mL)}} \times 100$
For student $C$:
$\text{Mass by volume } \% = \frac{50\,g}{100\,mL} \times 100 = 50\%$
Students $A$ and $B$ did not make the correct solution because they added $50\,g$ of $NaOH$ to $100\,mL$ or $100\,g$ of solvent,which results in a total solution volume greater than $100\,mL$.