Institution: Bangladesh Public Service Commission (BPSC) Exam: 17th BCS Preliminary Exam 1996
Topic: Correction (Sentence Correction), Subject: English Grammar (ইংরেজি গ্রামার)

Best Answer Explained in Below-:

The correct answer is D: "The Nile is the longest river in Africa." This sentence is grammatically accurate and conveys the intended meaning clearly. It includes the definite article "the" before "longest river," which is necessary because we are identifying a specific river—the Nile—as the longest in Africa.

Let's examine the options further:

- Option A: "The Nile is longest river in Africa." While it is almost correct, it lacks the article "the" before "longest river." It should read "the longest river."

- Option B: "The Nile is longest river in the Africa." This option incorrectly includes "the" before “Africa,” which is unnecessary. "Africa" is a proper noun and does not require a definite article.

- Option C: "Nile is longest river in Africa." This option is missing the definite article "the" before "Nile" and also "the" before "longest." Proper usage requires that we say "the Nile" and "the longest river."

In conclusion, Option D is comprehensive and follows proper English grammar rules, making it the correct choice.