The correct option is C Line
mm2 is the unit of measurement of area. e.g. area of a mobile screen in 45 mm2
cm is the unit of measurement of length e.g. length of a pencil is 6 cm
m3 is the unit of measurement of volume e.g a glass can hold 0.02 m3 of water.
Line is not a unit of measurement since it is a 1 D shape.
Therefore, option (c.) is the correct answer.