The plural form of any noun is the word used when there is more than one entity of any certain something.
The two given words are ‘datum’ and ‘radius.’ We are asked to use one of them, so we will use ‘radius.’
The plural form of ‘radius’ is ‘radii.’
So, a sentence using ‘radii’ will be – The sum of radii of two circles is greater than the third one.
Final Answer: A sentence using the plural form of ‘radius’ is – The sum of radii of two circles is greater than the third one.