Step 1: Recognize the correlative conjunction pattern.
The sentence begins with "not only," which is always paired with "but also" to form the correlative conjunction "not only...but also." These two parts always appear together.
Step 2: Apply the rule.
"Not only" paired with "does," "or," or "and" does not form a standard grammatical structure. Only "but also" completes this pair correctly.
Step 3: Confirm the answer.
"She not only studies well but also plays tennis excellently" is grammatically perfect. \[ \boxed{\text{but also}} \]