Which of the following materials is commonly used for bonding hair extensions?

The use of glue in bonding hair extensions is a widely accepted method in the hair extension industry. Glue provides a strong adhesive property that can securely attach hair extensions to natural hair. It is particularly effective for various types of hair extensions, including bonding or fusion methods where the extensions are glued directly to the client's hair. This method enables the extensions to blend seamlessly with natural hair, providing a natural look and feel when applied correctly.

Other materials, while they might be used for securing hair extensions in different contexts, do not offer the same strong adhesion that glue provides for permanent or semi-permanent installations. Sewing thread is typically used for weft extension applications but does not bond hair in the same way glue does. Hair gel serves a different purpose in hairstyling and has no adhesive properties suitable for extensions. Double-sided tape can be used for some temporary installations but is less common than glue for traditional bonding methods due to durability concerns.