The sentence says <em>"the author's intent is..."</em> and we require an adverb to fulfil the blank because adverbs modify verbs and the word "demostrated" is a verb. The best option is the adverb "thoroughly" which means completely. So the sentence would be <em>"The author's intent </em><em>is thoroughly demostrated</em><em> in the following paragraph."</em>
This statement is jargon, and jargon is the language used by certain professions, such as lawyers, doctors, etc. So having this in mind, we can see that the sentence clearly used medical terms, so it has to be jargon.
