Answer:
In the secluded pond, mallard ducks swim.
Step-by-step explanation:
Sentence fragments are groups of words looking like a sentence in the sense that they begin with a capital letter and end with a punctuation mark (like a period) but that, in reality, are not sentences because they lack a subject, a predicate or both and thus, do not express complete thoughts. To correct fragments, then, we must include them within a sentence.
In the secluded pond, mallard ducks swim is a construction having a subject: mallard ducks, and a predicate, which is all the rest that is stating something about the subject; and expressing a complete thought on its own, and therefore, it is the sentence that best corrects the fragment.