The correct option is 'The, the'. We use the definite article 'The' before the names of unique geographical features like rivers (The Nile). We also use 'the' before superlative adjectives (the longest) because there is only one river that holds this unique position. Using 'a' is incorrect in both blanks because 'Nile' is a specific, proper name, and 'longest' represents a unique, supreme quality.