The correct answer is D. no article.
The sentence “Nelson was a boy without fear” is grammatically correct, but it is not idiomatic. In English, we do not usually use articles with abstract nouns like “fear.” We would say “Nelson was a brave boy” or “Nelson was fearless.”
The sentence “Nelson was an boy without fear” is not grammatically correct. The article “an” is used before singular nouns that begin with a vowel sound. The word “boy” begins with a consonant sound, so we cannot use the article “an.”
The sentence “Nelson was the boy without fear” is also not grammatically correct. The definite article “the” is used to refer to a specific person or thing that has already been mentioned. In this sentence, we have not yet been introduced to Nelson, so we cannot use the definite article “the.”
Therefore, the correct answer is D. no article.