The correct answer is option D. A possessive pronoun.
The word <em>"its" </em>consists of the pronoun <em>it</em>, in the case of the sentence, it refers to Miller's pool as an object, and the liner as the property belonging to the pool - the existence of the property demands the addition of the letter "s" to the pronoun "it".
The rest of the options cannot be correct since:
A. <em>its </em>is not a unique element in the sentence. Therefore, it is not capitalized, nor it represents a proper noun.
B. In order for <em>its </em>to be a contraction, an apostrophe must be added - as in <em>"it's". </em>However, in the context of the sentence, <em>its </em>represents the property of an object, rather than a definition of Miller's pool, which would read as "it is liner was torn into two pieces", lacking coherence.
C. The particular or simple form of <em>its</em> is the pronoun <em>"it"</em>, which can't be used to represent property in the sentence.
