This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
A 24 year-old cook in a hostel mess suffered from enteric fever 2 years back. The chronic carrier state in this patient can be diagnosed by - 
Select and explain. 
A. Vi agglutination test., B. Blood Culture in Brain Heart infusion broth., C. Widal test, D. All of the above
A: Vi agglutination test.
Answer: A: Vi agglutination test. 
Explanation: Vi agglutinins of titer of > 1:10 is taken significant for diagnosis of Typhoidal carriers