When a person has episodes of hypomania that shift back and forth with mild depression for at least two years, he or she has what form of bipolar disorder?

To determine the answer, let's break down the options:

A. Bipolar II Disorder: This form of bipolar disorder involves recurrent episodes of major depression and hypomania. However, for a diagnosis of Bipolar II Disorder, the person must have experienced at least one episode of major depression and at least one episode of hypomania. Therefore, this option does not fit the description given in the question.

B. BP-NOS (Bipolar Disorder Not Otherwise Specified): This category is used when the symptoms do not meet the criteria for any specific form of bipolar disorder. It is a catch-all diagnosis for cases that don't fit neatly into other categories. While it is a possibility, the question specifically mentions episodes of hypomania and mild depression persisting for a minimum of two years. This suggests a more specific form of bipolar disorder.

C. Bipolar I Disorder: This form of bipolar disorder involves episodes of mania and depression. Manic episodes are more severe than hypomanic episodes and can significantly impair daily functioning. Since the question mentions only episodes of hypomania and mild depression, Bipolar I Disorder does not align with the description given.

D. Cyclothymic Disorder: This form of bipolar disorder involves numerous periods of hypomanic symptoms and mild depressive symptoms that persist for at least two years (or one year in children and adolescents). The key here is that the episodes of hypomania and depression are not severe enough to meet the criteria for a diagnosis of bipolar I or II disorder. Therefore, the person described in the question likely has Cyclothymic Disorder.

Hence, the correct answer is D. Cyclothymic Disorder.