Which of the following is a donor branch?

A donor branch refers to the branches within the military that contribute personnel to other branches based on their specific needs. The correct response indicates that none of the provided options—Armor, Infantry, or Medical Services—qualify as donor branches.

In this context, it's important to understand that while each of these branches plays a crucial role in military structure and operations, they do not function as donor branches in the same capacity as others might, such as certain support or administrative branches that are organized primarily for the purpose of augmenting forces in other specialties. The listed branches have defined roles and do not interchange personnel with those responsibilities as a primary function, thus leading to the conclusion that none of them fit the role of a donor branch.