Final answer:
Projective identification is the defense mechanism most likely used by individuals with borderline personality disorder, where they project their unacceptable feelings onto others, who then act by those projections.
Step-by-step explanation:
The defense mechanism MOST likely used by clients with a diagnosis of borderline personality disorder is projective identification. This is a complex mechanism that involves an individual projecting their unacceptable thoughts, feelings, or impulses onto another person. That other person may then be provoked into acting under the projected beliefs or emotions, which the first individual then identifies with. For example, a client might project feelings of anger onto a therapist, who might become frustrated in turn, validating the client's original feelings of being surrounded by anger. This differs from other defense mechanisms such as rationalization, in which individuals justify behaviors by substituting acceptable reasons for less-acceptable real reasons, or sublimation, where socially unacceptable impulses are transformed into socially acceptable actions.