Explanation:
In casting, defects can occur due to various reasons such as improper design, material selection, process parameters, etc. These defects can affect the quality and integrity of the final product. The given options are all common types of casting defects, except for decarburization.

Decarburization:
Decarburization is not a casting defect but rather a surface condition that occurs during the heat treatment process. It refers to the loss of carbon content from the surface layer of a material, typically steel, due to exposure to high temperatures in an oxidizing atmosphere. This results in a decrease in the hardness and strength of the material.

Other Casting Defects:
1. Hot Tear:
Hot tear or hot cracking is a defect that occurs due to the presence of high tensile stresses during solidification. It is characterized by cracks or tears in the casting, usually in the area of highest thermal stress. This defect is more common in metals with low ductility.

2. Blow Hole:
A blow hole is a type of porosity defect that occurs due to the entrapment of gases during the solidification process. These gases can be released from the molten metal as it solidifies, resulting in voids or cavities in the casting. Blow holes can affect the mechanical properties and surface finish of the final product.

3. Scab:
Scab defects occur when the sand mold or core surface breaks away and becomes embedded in the casting. This can happen due to excessive moisture content in the mold or poor sand compaction. Scabs can result in surface roughness and reduced dimensional accuracy.

In summary, while hot tear, blow hole, and scab are common casting defects that can affect the quality and integrity of the final product, decarburization is not a casting defect but rather a surface condition that occurs during heat treatment.