Arrange the following lengths in their increasing magnitude: 

1 metre, 1 centimetre, 1 kilometre,1 millimetre.

The SI unit of length is metre (m).

Each metre (m) is divided into 100 equal divisions, called centimetre (cm) and each centimetre has ten equal divisions, called millimetre (mm). 

i.e., 

$1 m = 100 cm$

and $1 cm = 10 mm$

On the other hand larger unit of length is expressed as kilometre (km). 

i.e., 

$1 km = 1000 m$

Thus the increasing magnitude of length is as follows:

1 millimetre < 1 centimetre < 1 meter < 1 kilometre