Fill in the blank in the sentence given below with the correct option. This…

The correct answer is Option D (fairly).
In the sentence 'This article on millennials is ________ interesting,' we need an adverb to modify the adjective 'interesting.'
The word 'fairly' is an adverb meaning 'to a moderate degree,' which fits perfectly here.
Option A (fair) is an adjective and cannot modify another adjective like 'interesting.'
Options B (fairer) and C (fairest) are comparative and superlative forms of the adjective 'fair,' which also cannot modify an adjective.
Therefore, only the adverb 'fairly' correctly completes the sentence by describing how interesting the article is.