the GPE is 1/2 the unit of measure used to make the measurement or you say 1/2 the most precise unit
12.3 L has 0.1 as the most precise unit of measure, so 1/2 of 0.1 is 0.05 L which is c. above
look at it this way: 12.3 can be any number between 12.25 and 12.34 rounded to the nearest tenth
any number from 12.25 to 12.34 rounded to the nearest tenth gives you 12.3, so the GPE that you can possibly make is 0.05; if the number was 12.24 it would round down to 12.2 not up to 12.3 and if the number was 12.35 it would round up to 12.4 not round down to 12.3, so again the greatest possible error that you can make is 0.05
