Which of the following must not touch the isolated natural eyelash?

The correct answer is that left or right neighboring lashes must not touch the isolated natural eyelash. When applying eyelash extensions, it is crucial to ensure that each extension is applied to an individual natural lash to achieve the best results in terms of both appearance and longevity.

When neighboring lashes touch the isolated natural lash, this can create issues such as lash bonding or fusion, which may lead to improper lash extension application and ultimately result in damage or breakage of the natural lashes. It also decreases the ability for each extension to move independently, which can cause discomfort or an unnatural look. Therefore, maintaining a clean and isolated environment for each lash is essential for successful application.

The other options involve components that can be utilized safely during the application process. The lash extension, adhesive, and tweezers are tools and materials that are designed to come into contact with the isolated lash during the application; however, proper technique and care must be exercised to prevent them from affecting the natural lash's health or the quality of the extension application.