**Core Concept**
The question is testing the student's knowledge of the diagnostic approach to typhoid fever, specifically the role of different investigations in the different stages of the disease.

**Why the Correct Answer is Right**
In typhoid fever, the Widal test is used to diagnose the disease by detecting the presence of H and O antibodies in the patient's serum against Salmonella Typhi. However, the Widal test becomes positive only after 7-10 days of the onset of fever, and it may remain positive for several months after recovery. Therefore, in a patient who presents after 15 days of fever, the Widal test may not be the best choice for diagnosis. However, the patient's clinical presentation, along with other investigations like stool culture, blood culture, or imaging studies, may still suggest typhoid fever.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.
* **Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The Widal test is not a definitive diagnostic test for typhoid fever and should be interpreted in conjunction with clinical findings and other investigations.

**Correct Answer:** D.