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

The correct option is 8500 mm, 80 dm 8 cm, 800 cm 8 mm, 8 m.

To arrange the given lengths in decreasing order, we first convert all measurements to a single standard unit, such as millimeters (mm).

Recall the relationships between unit conversions:
1 meter (m) = 1000 millimeters (mm)
1 decimeter (dm) = 100 millimeters (mm)
1 centimeter (cm) = 10 millimeters (mm)

Now, let's convert each length step-by-step:

1. 8500 mm is already in millimeters:
$8500\text{ mm}$

2. 80 dm 8 cm:
$80\times 100\text{ mm}+8\times 10\text{ mm}=8000\text{ mm}+80\text{ mm}=8080\text{ mm}$

3. 800 cm 8 mm:
$800\times 10\text{ mm}+8\text{ mm}=8000\text{ mm}+8\text{ mm}=8008\text{ mm}$

4. 8 m:
$8\times 1000\text{ mm}=8000\text{ mm}$

Now, compare the lengths in millimeters:
$8500\text{ mm}>8080\text{ mm}>8008\text{ mm}>8000\text{ mm}$

Arranging the original values in decreasing order:
8500 mm, 80 dm 8 cm, 800 cm 8 mm, 8 m