Answer:
A
Step-by-step explanation:
A dictionary would be the best type of source to use when looking for the root work of navigable. Dictionaries customarily give the definition of a word, its pronunciation, an example of the use of the word in a sentence, and the origin of the word.
An encyclopedia would be more appropriate to define shipping, and a textbook would likely say very little about the origins of a word and would instead use it in the context of the textbook's content.