Which of the following sentences is grammatically correct?
A. The student wrote too slow to finish the exam, but he has always been a slow writer.
B. The student wrote too slow to finish the exam, but he has always been a slowly writer.
C. The student wrote too slowly to finish the exam, but he has always been a slowly writer.
D. The student wrote too slowly to finish the exam, but he has always been a slow writer.
In this sentence, the adverb "slowly" is used correctly to modify the verb "wrote," indicating the manner in which the student wrote. Additionally, the adjective "slow" is appropriately used to describe the writer in the second part of the sentence.
Therefore, the Correct Answer is D.