Aptitude : lack of skill ::

a. injunction : disunity

b. pertinacity : stubbornness

c. subjugation : dominate

d. colloquialism : formal speech

Since 'aptitude' and 'lack of skill' are antonyms, they have opposite meanings, we are looking here for a pair of words which also have opposite meanings. Since A, B, and C are basically synonymous words, that is, they have the same meaning, the correct answer is D, because 'colloquialism' and 'formal speech' are opposites, because 'colloquialism' mean 'informal speech'.