**Core Concept**
Tyndallisation is a laboratory technique used to eliminate microorganisms, particularly spores, from a liquid or solid substance. This method involves heating the substance to a high temperature, followed by a cooling period, and then repeating the process several times to ensure the complete destruction of any remaining microorganisms.

**Why the Correct Answer is Right**
The process of Tyndallisation is based on the principle of differential heat resistance, where spores are more resistant to heat than vegetative cells. By repeatedly heating and cooling the substance, the vegetative cells are killed, while the spores are not immediately killed and germinate only after the cooling period. This allows for the removal of spores from the substance. The process is named after John Tyndall, who first described it in the 19th century.

**Why Each Wrong Option is Incorrect**
**Option A:** Sterilization by autoclaving - While autoclaving is a method of sterilization, it is not the same as Tyndallisation, which specifically involves repeated heating and cooling cycles.
**Option B:** Pasteurization - Pasteurization is a method of reducing the number of microorganisms in a substance, but it does not involve repeated heating and cooling cycles like Tyndallisation.
**Option C:** Filtration - Filtration is a method of removing microorganisms from a substance by passing it through a filter, but it does not involve heating or cooling.

**Clinical Pearl / High-Yield Fact**
Tyndallisation is an important technique in microbiology and is used in various fields, including food safety, pharmaceuticals, and biotechnology. It is a critical step in ensuring the quality and safety of products.

**Correct Answer:** D.