Final answer:
Heather has been feeling depressed and listless lately. For Heather to be diagnosed with major depression, these symptoms must last for at least 2 weeks (option a).
Step-by-step explanation:
For Heather to be diagnosed with major depression, the symptoms she is experiencing must last for at least 2 weeks. Major depressive disorder is characterized by a severe depressed mood and a cluster of additional symptoms such as changes in appetite and sleep, loss of interest in enjoyable activities, difficulty concentrating, feelings of worthlessness, and possibly self harming thoughts. To be clinically diagnosed, these symptoms must not only be present for at least two weeks, but they must also represent a change from previous functioning and cause significant distress or impairment in daily activities.
Hence, the answer is option a.