To our right, was . . . . . . . . marshy pond.

The correct answer is D. no article.

The article "a" or "an" is used before singular nouns that refer to something that is not specific. In this case, the noun "marshy pond" is not specific. It could be any marshy pond, not a particular one. Therefore, we do not use an article.

The article "the" is used before singular nouns that refer to something that is specific. In this case, the noun "marshy pond" is not specific. It could be any marshy pond, not a particular one. Therefore, we do not use "the".