Final answer:
Option 2. is correct. Plastic extensions can be damaged by high heat, resulting in them cutting, breaking or melting, because they have a lower melting point than materials like metal or glass.
Step-by-step explanation:
The extension material that may cut or break the hair and melt with the application of excessive heat is plastic. Plastic extensions are synthetic and can be particularly sensitive to high temperatures from styling tools such as curling irons and straighteners. This is because plastic has a lower melting point compared to materials like metal or glass. Therefore, it's important to use lower temperature settings when styling synthetic hair extensions to avoid damage.
The extension material that may cut or break the hair and melt with the application of excessive heat is plastic. When plastic is exposed to high temperatures, it can soften and deform, causing it to cut or break the hair. This can happen with some types of hair extensions that are made from synthetic or low-quality materials.