The green plant with red berries commonly hung at Christmas, beneath which people traditionally kiss, is mistletoe.
Mistletoe is not one single plant but a group of partially parasitic plants that grow on the branches of trees. European mistletoe, Viscum album, has evergreen leaves and white berries rather than red ones; red-berried mistletoe species are found in other regions. The quiz description refers to the familiar Christmas decoration and tradition.
The kissing custom developed in Britain and became especially popular during the eighteenth and nineteenth centuries. Earlier European beliefs also connected mistletoe with fertility, vitality, protection, and good fortune. A Victorian variation said that a berry should be removed after each kiss, with the kissing ending when no berries remained.
Mistletoe is often confused with holly, another Christmas plant with berries. Holly has spiny leaves and red berries, while mistletoe is usually seen hanging from a doorway or ceiling. Some mistletoe berries are toxic, so the decorative plant should not be eaten.