, Tthe noun is plural (‘two circles’). However, the word ‘radius’ is singular, which is inconsistent because the verb ‘are’ also denotes the existence of a plural noun. Therefore, ‘radius’ should get replaced by a plural noun.
Let us analyze the choices given to us during this question:
Option (a.), 'no change', suggests that the word ‘radius’ within the sentence should remain because it is, which the sentence is thus correct.
Therefore, option (a.) is inaccurate as ‘radius’ should get replaced by a plural noun.

Option (b.), ‘arc', refers to a noun that's singular.
Therefore, option (b.) is inaccurate as ‘radius’ should get replaced by a plural noun.

Option (c.), ‘diameter', refers to a noun that's singular.
Therefore, option (c.) is wrong as ‘radius’ should get replaced by a plural noun.

Option (d.), ‘radii', refers to a noun that's plural.
Therefore, option (d.) is correct as ‘radius’ should get replaced by a plural noun.

Note: during this question, there should be consistency in terms of numbers within the given sentence. that's why we've to search out the choice which includes a plural noun.