Step 1: Understanding the Widal Test.
The Widal test is a diagnostic test used to detect the presence of antibodies against the bacteria *Salmonella typhi*, the causative agent of typhoid fever. It is commonly used in clinical laboratories to diagnose typhoid.
Step 2: Analysis of options.
- (A) Pneumonia: Pneumonia is caused by different pathogens like bacteria, viruses, or fungi, but not by *Salmonella typhi*. The Widal test is not used for pneumonia diagnosis.
- (B) Typhoid: Typhoid fever is caused by *Salmonella typhi*, and the Widal test is specifically used for diagnosing this disease.
- (C) Malaria: Malaria is caused by *Plasmodium* parasites, not *Salmonella typhi*, and is diagnosed through blood tests like the Rapid Diagnostic Test (RDT) or blood smear examination, not the Widal test.
- (D) Cholera: Cholera is caused by *Vibrio cholerae*, not *Salmonella typhi*, so the Widal test is not used for cholera diagnosis.
Step 3: Conclusion.
The Widal test is used to diagnose Typhoid fever, making option (B) the correct answer.

Study the chemical structure of an addictive drug shown below and answer the questions that follow.