226 grams.
First, let's calculate the volume of the block.
1.00cm * 4.00cm * 2.50cm = 10 cm^3
Now you multiple by the density. So:
10 cm^3 * 22.57 g/cm^3 = 225.7 g
So the mass of the block of osmium is 225.7 grams. But since we only have 3 significant figures in our least precise piece of data (the dimensions of the block), the result should be rounded to 3 significant figures, giving 226 grams.