Mass by volume percentage of 0.1 M NaOH aqueous solution is : 

0.1 M indicates that 0.1 mole of solute (NaOH) dissolved in 1 L or 1000 mL of solution.

Mass present in 1 mole of solute:

$$\begin{gathered} \mathrm{mol}=\frac{\mathrm{Mass}}{\mathrm{Molar} \mathrm{mass}} \\ 0.1 \mathrm{mol}=\frac{\mathrm{Mass}}{40 \mathrm{g}/\mathrm{mol}} \\ \mathrm{mass}=4 \mathrm{g} \end{gathered}$$

Mass/volume percentage is calculated as:

$\frac{4 \mathrm{g}}{1000 \mathrm{g}}\times 100=0.4\%$