Answer:
A dictionary.
Step-by-step explanation:
A dictionary is a reference book that provides the meaning of words, be it the definitions, word origins, or even the synonyms and antonyms. It also provides the parts of speech.
In order to find what part of speech is the word "synopsis", using a dictionary will help. The book or even the online versions contain all details about any word, and the online versions also provide the actual ways to pronounce any word. And the end result will be that a synopsis means a brief summary of any passage, text or anything.
Thus, the correct answer is a dictionary.