**Core Concept**
Mentalisation-based therapy (MBT) is a type of psychotherapy that focuses on improving an individual's ability to understand and interpret their own emotions and those of others. It is particularly effective in treating disorders characterized by unstable relationships, emotional dysregulation, and impulsivity.

**Why the Correct Answer is Right**
MBT was initially developed for treating borderline personality disorder (BPD). This disorder is marked by a pervasive pattern of instability in interpersonal relationships, self-image, and affects, and marked impulsivity. MBT aims to enhance mentalising skills in individuals with BPD, enabling them to better understand and manage their emotions, as well as develop more stable and adaptive relationships. The therapy involves a combination of individual and group sessions, with a focus on exploring the patient's mental states and understanding the underlying emotional processes that drive their behavior.

**Why Each Wrong Option is Incorrect**
**Option A:** Antisocial personality disorder is characterized by a disregard for others' rights, lack of empathy, and impulsivity. While individuals with antisocial personality disorder may benefit from various forms of psychotherapy, MBT is not specifically tailored for this condition.

**Option B:** Obsessive-compulsive personality disorder is marked by a pervasive pattern of preoccupation with orderliness, perfectionism, and control. MBT is not typically used to treat this disorder, as it focuses more on emotional dysregulation and interpersonal difficulties.

**Option C:** Avoidant personality disorder is characterized by social inhibition, feelings of inadequacy, and hypersensitivity to criticism. While individuals with avoidant personality disorder may benefit from certain forms of psychotherapy, MBT is not specifically designed for this condition.

**Clinical Pearl / High-Yield Fact**
Mentalisation-based therapy is an effective treatment for borderline personality disorder, with studies showing significant improvements in symptoms, relationships, and overall quality of life. It is essential for psychiatrists to be aware of this therapy and its applications in treating complex personality disorders.

**✓ Correct Answer: D. Borderline personality disorder**