**Core Concept**
Tyndallisation is a method of sterilizing liquids or media by repeatedly heating them to a temperature just below boiling, followed by cooling, to kill most microorganisms, particularly spores. It is based on the principle of intermittent exposure to heat, which allows for the destruction of heat-resistant spores through repeated cycles.

**Why the Correct Answer is Right**
Tyndallisation involves heating the medium to a temperature (usually 60–70°C) for a period, then cooling it, and repeating this process multiple times. This intermittent heating prevents the formation of bacterial spores and effectively reduces microbial load. Unlike autoclaving, which uses high-pressure steam, Tyndallisation is a non-continuous sterilization method, making it a form of **intermittent sterilization**. It was historically used for sterilizing nutrient media before autoclaving became widespread.

**Why Each Wrong Option is Incorrect**
Option B: Pasteurisation involves heating to kill pathogens but not spores, and is used for food preservation, not sterilization.
Option C: Boiling kills most microbes but not spores and is insufficient for sterilization.
Option D: Autoclaving uses high-pressure steam (121°C, 15 psi) to achieve sterilization in minutes, a continuous process, not intermittent.

**Clinical Pearl / High-Yield Fact**
Tyndallisation is now obsolete in modern microbiology labs due to the efficiency and reliability of autoclaving, but understanding it is essential for grasping historical sterilization techniques and the difference between sterilization and disinfection.

✓ Correct Answer: A. intermittent sterilization