Since 1 m = 100 cm, then 1 m³ = 100 × 100 × 100 = 1,000,000 cm³. So 68 m³ = 68000000 cm³. In mensuration, area and volume conversions are different from simple length conversions. For square units, square the conversion factor; for cubic units, cube the conversion factor. So the correct choice is 68000000 cm³.