Since, 1 kilometre = 1000 metre
So, 1 kilometre > 1 metre
And 1 metre = 100 centimetre
So, 1 metre > 1 centimetre
And 1 centimetre = 10 millimetre
i.e 1 centimetre > 1 millimetre
Thus,
Ascending order of length:
1 millimetre < 1 centimetre < 1 metre < 1 kilometre