Step 1: Understanding the Sentence
This sentence, a quote from Thomas Jefferson, defines the primary goal of good government. The first blank needs a pronoun to refer to "human life and happiness". The second blank needs an adjective to emphasize that this is the \textit{sole} primary objective.
Step 2: Evaluating the Options
First Blank: The pronoun must refer to "human life and happiness". Since humans are being referred to, the plural possessive pronoun "their" is appropriate, referring to the destruction of humans. "Its" would be singular and impersonal. "There" is an adverb of place. So, "their" is the best fit.
Second Blank: The phrase "the first and ________ object" is meant to be emphatic. The word "only" fits perfectly, meaning it is the single most important objective. "Certain" could fit, but "only" provides a stronger, more definitive meaning that aligns with the tone of the quote.
Let's check the pairs:
(A) Their, only: "not their destruction" and "the first and only object". This pair works perfectly.
(B) Only, their: "not only destruction" changes the meaning.
(C) Its, certain: "Its" is a less suitable pronoun than "their". "Certain" is a weaker adjective than "only".
(D) There, only: "There" is incorrect.
Step 3: Final Answer
The pronoun "Their" correctly refers to humans, and the adjective "only" correctly emphasizes the singular importance of the government's object. Therefore, option (A) is the correct answer.