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

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.