Final answer:
The hair addition method that attaches hair extensions to natural hair using a bonding agent and heat is called fusion.
Step-by-step explanation:
The hair addition method that uses a bonding agent activated by a heating element to affix loose hair fiber to small sections of the client's own hair is known as fusion. This method involves taking strands of hair extensions and attaching them to tiny sections of the natural hair using a bonding substance, usually a type of glue or keratin. The bond is then sealed with heat from a special tool, which allows for a durable extension that lasts until the hair naturally sheds or the bond is broken.