The correct answer is D. A dictionary
Step-by-step explanation:
In general terms, a dictionary is a source or document that lists the words of a language following alphabetical order and includes the definition of the words lists, besides further information about words such as the type of word, origin or pronunciation. Due to this, if you want to locate the definition for the word "vital" or any other word the best is to use a dictionary and more specifically and English dictionary. On the other hand, other sources are not appropriate because a thesaurus is used for finding synonyms, an atlas is used for finding maps and an almanac describes annual events.