The idea here is that you need to convert the concentration of sodium hydroxide from moles per cubic decimeter to grams per cubic decimeter.
In order to to that, you need to use the molar mass of sodium hydroxide, which is equal to
So, if you take a sample of this solution that has a volume of
Use the molar mass of sodium hydroxide to convert the number of moles to grams
#0.10 color(red)(cancel(color(black)("moles NaOH"))) * "39.997 g"/(1color(red)(cancel(color(black)("mole NaOH")))) = "3.9997 g"#
Since this represents the number of grams of sodium hydroxide present in
#color(darkgreen)(ul(color(black)("concentration NaOH = 4.0 g dm"^(-3))))#
The answer must be rounded to two sig figs, the number of sig figs you have for the concentration of the solution in moles per cubic decimeter.