Answer:
Mostly millimeter
Explanation:
Length has several unit like m,cm, mm, feet, miles, inches, km etc
we measure the length in units which is more understandable or readable. like thickness of mobile is around 3mm to 1cm or we can even measure in inches like .5inch but if we measure it in meter or feet it will be difficult to understand, read and write similarly the unit of length most suitable for measuring the height of a backyard tree is meter or feet because the length of tree is around 10 feet to 25 feet or 2meter to 6meter if we measure the length in cm or mm or in inches then it will be difficult to understand, read and write.