The correct answer is D. no article.
The article “a” or “an” is used before singular countable nouns that are not specific. In this case, the noun “sight” is singular and countable, but it is not specific. The speaker does not know what the unusual sight is, so it is not preceded by an article.
The article “the” is used before singular countable nouns that are specific. In this case, the noun “sight” is singular and countable, but it is not specific. The speaker does not know what the unusual sight is, so it is not preceded by the article “the.”
No article is used before plural countable nouns, uncountable nouns, and proper nouns. In this case, the noun “sight” is singular and countable, but it is not specific. The speaker does not know what the unusual sight is, so it is not preceded by an article.