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
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