47.201 cm^3
Remember that density is defined as mass per volume. So you can divide your known mass by your known density and get the volume occupied. So
1000 g / 21.186 g/cm^3 = 47.20098178 cm^3
Therefore the volume of the standard kilogram is
47.20098178 cm^3
Now the question is "How many significant digits do I have?". For this case, the mass of the standard kilogram is for all practical purposes known to be exactly 1 kilogram to as many significant digits as you want because it is the definition of the kilogram. So we have 5 significant figures because that's the number of significant figures we have available for the density. So round the result to 5 significant figures, giving 47.201 cm^3
