I could not find the paragraph five that is missing in our question but I assume that the right answer is B) because of the word ''navigable'' meaning.
''Navigable'' word is mostly used when writer or speaker are describing ability or allowance of ships, boats and other water vessels to move and sail trough sea or waterway. Similar meaning of it is the word ''passable''.
Because of its meaning I assume that the answer B) is the most appropriate because it describes that the waterway is deep and wide for ships so we can also say that is passable.
