Since 1 m = 100 cm, then 1 m² = 100 × 100 = 10,000 cm². Therefore, 78 m² = 78 × 10,000 = 780000 cm². 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 780000 cm².