For each question below, a statement with a highlighted phrase is given. Choose the most appropriate replacement from the given options which could replace the highlighted phrase to make the statement grammatically and contextually correct. If the highlighted phrase is already correct then choose option "No replacement required" as your response.

Although she exercises regularly, but a balanced diet is crucial to stay fit.

The correct option is a balanced diet is.

Detailed Explanation:
1. Redundancy of Conjunctions: The sentence begins with the subordinating conjunction "Although", which introduces a clause of concession ("Although she exercises regularly"). In standard English grammar, when a sentence starts with "Although", a coordinating conjunction such as "but" or "then" should not be used to connect the main clause. Using both "Although" and "but" creates grammatical redundancy.
2. Subject-Verb Agreement: The subject in the main clause is "a balanced diet", which is a singular noun phrase. Therefore, it requires the singular verb "is" rather than the plural verb "are".
3. Correct Sentence Structure: Removing "but" yields the grammatically sound and contextually meaningful sentence: "Although she exercises regularly, a balanced diet is crucial to stay fit."

Thus, the highlighted phrase "but a balanced diet" should be replaced by "a balanced diet is".