Option A: In the given sentence, the noun 'two circles' is plural.
So, there should be a consistency in terms of number in a sentence. The
word 'radius' is singular, it refers to only one radius, whereas
'circles' is plural. Hence this word must be changed and option A is
incorrect.
Option D: 'Radii' is the plural form of 'radius' and hence agrees with the rest of the sentence in terms of number. Hence option D is correct.
Options B and C: 'Arc' and 'diameter' are both singular nouns and, like 'radius', do not agree with the sentence in number. Hence these options are incorrect.