The correct answer is Humification.

Decomposition in soil is a complex process involving the breakdown of organic matter by various organisms such as bacteria, fungi, and invertebrates. Humification is a key part of this process. It refers to the transformation of partially decomposed organic matter into a stable, dark-brown, amorphous substance called humus. Humus is crucial for soil structure, fertility, and water retention. Let's examine why the other options are incorrect:

• Crystallisation: This is a process of forming crystals, typically from a solution, and is not directly related to the decomposition of organic matter in soil.
• Defragmentation: This term usually refers to the process of consolidating data or files on a computer; it has no relevance to soil decomposition.
• Oxidation: While oxidation is involved in some aspects of decomposition (like the breakdown of certain organic molecules), it doesn't define the overall process of transforming organic material into humus. Humification is a more encompassing and specific term describing the formation of humus, a crucial component of healthy soil.

In summary, humification is the central process that describes the conversion of organic matter into humus during soil decomposition, a critical step in nutrient cycling and soil health. The other options are either irrelevant or only represent a small part of the larger decomposition process. Therefore, humification best answers the question on decomposition in the soil.