Correct option is B
Dysthymia (Persistent Depressive Disorder) = chronic depressive symptoms for ≥2 years without full major depressive episodes.
Explanation of options: (a) ✘ Cyclothymia = fluctuating hypomania + mild depression. (b) ✔ Correct; long-term subthreshold depression = dysthymia. (c) ✘ Bipolar II requires hypomania + major depression. (d) ✘ Somatization = multiple physical complaints, not persistent low mood.