Choose the measurement that is most precise.
a. 12 mm
c. 1.2 m
b. 12 cm
d. 23 m

Think about how big each of these really is.

Precise is just how small a thing you've measured (in this question, scientists like to get kinda funky with definitions, but you'll encounter that in University)

Take a one meter ruler. To measure 23m (d) you'll need to put 23 of them end to end, that's huge! To measure 1.2m you just need 2 rulers, and you'll only use a fifth of the second one. Getting smaller, but let's keep going. To measure 12cm you only need one ruler, 100cm is 1m so 12cm is a small part of one ruler.

Lastly 10mm = 1cm, so 12mm = 1.2cm = 0.012m, your ruler probably only has marks every centimeter, so you'd be measuring something tiny, about the size of a fly or something.
 

So to express them all in meters makes the answer quite clear.

23m>1.2m>0.12m>0.012m

0.012 is clearly the smallest, and most precise measurement, the answer must be a