D. Since immunity requires a few weeks to developed, her "reaction" was most likely a real case of influenza acquired elsewhere.

Yes! The most likely explanation for this patient's post-vaccination illness is a coincidental case of influenza. The patient was likely immunized too late while seasonal influenza had already starting to circulate. Since the immunity from the vaccine does not kick in for at least 2 weeks, the patient may have acquired influenza during the interval between vaccination and the development of immunity.