A metre ruler is graduated in :

1. m
2. cm
3. mm
4. km

mm

Reason — A metre ruler is 1 metre long and is divided into 100 equal parts where each part is a centimetre and each centimetre is further divided into 10 equal parts where each small part is a millimetre. Thus, a metre ruler is marked in centimetre and millimetre and has a small division of 1 mm so it can measure a length correct up to 1 mm.