Final answer:
Beatrice's symptoms align most closely with Major depressive disorder, which is characterized by a significant loss of interest in previously enjoyed activities, sadness, sleep disturbances, concentration difficulties, and low energy.
Step-by-step explanation:
Based on Beatrice's symptoms, which include a loss of interest in activities she once enjoyed, feelings of sadness and frustration, trouble sleeping and concentrating, as well as low energy, the most fitting diagnosis from the provided options would be C. Major depressive disorder. While conditions like attention-deficit/hyperactivity disorder (ADHD) and bipolar disorder also involve issues with concentration and energy, the defining feature for Beatrice is her marked decrease in enjoying activities, which is characteristic of depressive disorders. Furthermore, whereas bipolar disorder would include periods of mania or hypomania, the scenario provided does not mention these types of episodes.