What effect has to occur for pyrolysis to take place?

Pyrolysis refers to the thermal decomposition of materials at elevated temperatures in the absence of oxygen. This process occurs when organic materials, such as wood or biomass, break down due to exposure to heat. For pyrolysis to take place, it is essential that the material is heated sufficiently to cause changes in its chemical structure without the presence of oxygen, which prevents combustion.

The process involves the breaking down of long-chain molecules into smaller ones, typically producing solid, liquid, and gaseous products: char, tar, and various gases, respectively. This defines pyrolysis and sets it apart from other thermal processes like combustion, which requires oxygen and leads to flames.

Given this understanding, the correct choice highlights the necessity of heat as the sole driving force behind pyrolysis, emphasizing that it does not involve reactions with oxygen, ignition by flames, or interactions with moisture. The other options describe different processes or requirements that do not capture the essence of pyrolysis itself.