**Core Concept**
Leprosy immunoprophylaxis aims to prevent the disease caused by Mycobacterium leprae. Immunization is crucial, especially in high-risk areas, to reduce the incidence of leprosy.

**Why the Correct Answer is Right**
The ICRC (International Committee of the Red Cross) bacillus, also known as M. leprae, is the causative agent of leprosy. Killed M. leprae bacillus vaccines have been developed and tested for their efficacy in preventing leprosy. These vaccines work by stimulating the immune system to recognize and respond to the M. leprae antigens, thereby providing immunity against the disease. This approach is especially important in areas with high leprosy prevalence, as it can help reduce the transmission of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** MMR (Measles, Mumps, and Rubella) vaccine is not related to leprosy immunoprophylaxis. It protects against measles, mumps, and rubella viral infections, not bacterial infections like leprosy.
**Option C:** Plague bacillus is the causative agent of plague, a different disease caused by Yersinia pestis, which is not related to leprosy.
**Option D:** Anthrax bacillus is the causative agent of anthrax, caused by Bacillus anthracis, which is also unrelated to leprosy.

**Clinical Pearl / High-Yield Fact**
The ICRC bacillus vaccine is not widely used due to concerns about its efficacy and potential side effects. However, it remains a valuable tool in specific contexts, such as in high-risk areas or during outbreaks. It is essential to recognize the limitations and potential risks associated with this vaccine.

**✓ Correct Answer: B. Killed ICRC bacillus**