. . . . . . . . Sahara is in the northern part of Africa.

Answer: Option D

The definite article the is used before the names of unique geographical features such as deserts, rivers, and mountain ranges.

The Sahara is a specific desert, making it a unique geographical feature.

Using no article is incorrect because the name of a unique geographical feature requires the definite article the.

a and an are incorrect because they are indefinite articles and are not used with unique or specific geographical features.

Therefore, the correct answer is the.