Carborundum is used as an abrasive because it is extremely hard. Abrasives are materials that are used for grinding, polishing, or smoothing surfaces by removing small particles of the material being worked on. In order for an abrasive to be effective, it needs to have a greater hardness than the material being worked on.

Option 1, which states that carborundum has high thermal conductivity, is incorrect. Thermal conductivity refers to the ability of a material to conduct heat, and it is not a determining factor for its abrasiveness.

Option 2, which says that carborundum has a low coefficient of expansion, is also incorrect. The coefficient of expansion refers to how much a material expands or contracts when subjected to changes in temperature. This property is not relevant to its ability to function as an abrasive.

Option 3, stating that carborundum has high chemical stability, is also incorrect. While chemical stability is important in some applications, it does not directly relate to its abrasive nature.

Therefore, the correct answer is option 4, which correctly identifies carborundum`s extreme hardness as the reason for its use as an abrasive.