The Widal test is a diagnostic test used to identify the presence of antibodies in the blood that are specific to the bacteria causing typhoid fever. Typhoid fever is a bacterial infection caused by Salmonella typhi, and it is characterized by symptoms such as high fever, abdominal pain, headache, and diarrhea.

Option 1: Tuberculosis is a different infectious disease caused by the bacterium Mycobacterium tuberculosis. The Widal test does not diagnose tuberculosis, so option 1 is incorrect.

Option 2: Tetanus is not a bacterial infection that can be diagnosed by the Widal test. Tetanus is caused by the bacterium Clostridium tetani, and its diagnosis is typically based on clinical symptoms and history of exposure to the bacteria. So, option 2 is incorrect.

Option 4: Leprosy, also known as Hansen`s disease, is caused by infection with Mycobacterium leprae. The Widal test is not used to diagnose leprosy, so option 4 is incorrect.

Therefore, the correct answer is option 3: Typhoid. The Widal test is specifically used to diagnose typhoid fever by detecting the presence of antibodies against Salmonella typhi in the blood.