have any idea of the topic / module or anything like that?
The question, as stated, makes no sense. If a mathematical circle (and this appears to be a mathematical question) has radius 16 mm then the "true radius" is exactly 16 mm. There is no "range of values".
However, if this is a physical circle, measured by some device, then the convention is that, unless other information is given, the measurement is given to the nearest accurate possible measurement- here, to the nearest mm. Then the "true radius" could be from 16- .5= 15.5 mm to 16+ .5= 16.5 mm.