Which of the following will be the most suitable unit for determining a coin's thickness?

• Millimetre is the most appropriate unit to measure coin thickness because it is a small unit.
• Other units given in the options can also be used to measure the thickness of the coin, but because these are relatively big units, the thickness of the coin will be very small—almost zero—in comparison.
• For instance, one kilometre equals 1000000 millimetre, so if a coin is 2.2 mm thick, it will be expressed as 0.0000022 km.
• Light year and mile are even bigger units than the kilometre. Therefore, the thickness of the coin will be even smaller in these units.

Therefore, millimetre will be the most suitable unit.