It is a process where continuity and change are in consonance with each other, where the introduction of a new segment does not subsume the importance of the existing segments.

(I) is in consonance with one another

(II) are in consonant to one another

(III) is in consonance at each other

The given sentence is grammatically correct as the plural verb “are” is used in accordance to its plural subjects “continuity and change”. Moreover, the phrase “with each other” is used to frame a relation with the two. Hence the sentence doesn’t require any correction. The three given phrases in the options make no relevant substitution to the phrase given in bold in the sentence as they do not follow the correct grammar syntax required for the sentence. Hence 5 is the correct option.