Is it permissible to pluck a single hair that descends from the eyebrow to the eyelid and approaches the eyelashes, knowing that the eyebrow is thick, and that is without plucking the eyebrow?

Plucking (al-Namṣ) refers specifically to removing hair from the eyebrows, not other facial hair. There is no harm in trimming the excess length of an eyebrow hair that droops over the eyelid, if it causes discomfort, and this is not considered the forbidden plucking. However, plucking the hair from its root remains, in principle, prohibited.