In which of the following, lengths have been arranged in decreasing order?

Convert all to mm: 8500 mm = 8500 mm; 800 cm 8 mm = 8000 + 8 = 8008 mm; 80 dm 8 cm = 8000 + 80 = 8080 mm; 8 m = 8000 mm. So order is: 8500 > 8080 > 8008 > 8000, which is 8500 mm > 80 dm 8 cm > 800 cm 8 mm > 8 m. Checking option 1: 8500 mm (8500), 800 cm 8 mm (8008), 80 dm 8 cm (8080), 8 m (8000) - this gives 8500 > 8008, but 8008 < 8080, so option 1 is not strictly decreasing. Let me recheck option 4: 8500, 8080, 8008, 8000 - this IS decreasing. So option 4 is correct.