Choose the correct plural form to complete the sentence: The _____ of animals in the zoo is fascinating.

The correct answer is "species" because it is both the singular and plural form of the word, which refers to a particular group of animals or plants that belong to the same category. In English, certain nouns, particularly those borrowed from Latin or scientific terminology, do not change in their plural form. "Species" is one of those words; therefore, when referring to multiple types or groups of animals in the zoo, you still use "species."

The other options do not conform to standard English usage for this term. "Specieses" is not accepted in formal English as the plural form of "species." "Specie" refers to currency or money in the form of coins rather than the biological context of animals, and "speciess" is simply a misspelling, as there is no standard English plural form with that ending. Thus, "species" is the appropriate choice in the context of the sentence.