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) '$C$' 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 '$C$':
$\text{Mass by volume } \% = \frac{50\, g}{100\, mL} \times 100 = 50\%$
Student '$A$' prepared a solution where the total volume is likely greater than $100\, mL$ (as $50\, g$ of $NaOH$ occupies volume),and student '$B$' prepared a mass by mass solution,not mass by volume.