**Core Concept**
The question is testing the knowledge of the most effective drug against *Mycobacterium leprae*, the causative agent of leprosy. Leprosy is a chronic bacterial infection that primarily affects the peripheral nerves, skin, and mucous membranes.

**Why the Correct Answer is Right**
The most effective drug against *M. leprae* is **Rifampicin**. Rifampicin acts by inhibiting the bacterial DNA-dependent RNA polymerase enzyme, which is essential for the transcription of bacterial RNA. This leads to a decrease in the bacterial load and an improvement in the clinical symptoms of leprosy. Rifampicin is usually used in combination with other drugs, such as Clofazimine and Dapsone, to treat leprosy.

**Why Each Wrong Option is Incorrect**
**Option A:** **Isoniazid** is a first-line antitubercular agent, but it is not effective against *M. leprae*. It works by inhibiting the synthesis of mycolic acid in the mycobacterial cell wall.
**Option B:** **Ethambutol** is another antitubercular agent that is used in combination with Isoniazid to treat tuberculosis. It is not effective against *M. leprae*.
**Option C:** **Praziquantel** is an anthelmintic drug used to treat infections caused by flatworms and roundworms. It is not effective against *M. leprae*.

**Clinical Pearl / High-Yield Fact**
Rifampicin can cause a harmless but distinctive orange-red discoloration of bodily secretions, such as saliva, sweat, and tears, due to its lipophilic nature.

**Correct Answer:** C. Rifampicin.