**Core Concept**
Tuberculosis (TB) is a bacterial infection caused by Mycobacterium tuberculosis, affecting primarily the lungs but can also affect other parts of the body. The evening rise of temperature is a classic symptom of TB, indicating a systemic response to the infection.

**Why the Correct Answer is Right**
The definitive diagnostic test for TB is the **Gold Standard**, which is the **AFB (Acid-Fast Bacilli) smear microscopy**. This test involves examining a sputum sample under a microscope for the presence of acid-fast bacilli, which are characteristic of M. tuberculosis. The test is considered definitive because it directly detects the causative organism of TB.

**Why Each Wrong Option is Incorrect**
**Option A:** **Chest X-ray** is an important diagnostic tool for TB, but it is not definitive as it can show non-specific findings in TB, such as infiltrates or cavitations, which can also be seen in other lung conditions.

**Option B:** **Tuberculin Skin Test (TST)** is a diagnostic tool that detects the presence of antibodies against M. tuberculosis, but it is not definitive as it can give false-positive results in people who have been vaccinated with BCG or have been exposed to non-tuberculous mycobacteria.

**Option D:** **Mantoux test**, also known as the Tuberculin Skin Test, is a type of TST that involves injecting a small amount of tuberculin into the skin and observing the reaction after 48-72 hours. While it is a useful diagnostic tool, it is not definitive as it can give false-positive results.

**Clinical Pearl / High-Yield Fact**
The evening rise of temperature in TB is thought to be due to the evening peak in cortisol levels, which suppresses the immune response and allows the bacteria to multiply. This is why TB patients often experience a worsening of symptoms in the evening.

**Correct Answer:** C. AFB (Acid-Fast Bacilli) smear microscopy.