Which of the following is NOT a type of tree support system?

Trimming is the correct choice as it does not fall under the category of tree support systems. Instead, trimming refers to the practice of pruning branches or foliage for aesthetic purposes, health reasons, or to improve visibility and safety. It is mainly about shaping and maintaining the tree's appearance and growth rather than providing structural support.

In contrast, cabling, bracing, and guying are all methods specifically designed to support trees. Cabling involves installing flexible cables to help stabilize weak branches or tops, while bracing is typically used to provide additional support to tree trunks or limbs that may be at risk of splitting. Guying, on the other hand, is a technique used to secure a tree to the ground or other stable structures to prevent it from leaning or falling over. These methods are all focused on enhancing the structural integrity and safety of trees, distinguishing them clearly from trimming, which serves a different purpose.