**Core Concept**
Depression is a complex mood disorder characterized by a combination of emotional, cognitive, and physical symptoms. The classical triad of symptoms of depression helps clinicians diagnose and manage this condition by identifying key features such as mood disturbances, cognitive impairments, and somatic complaints.

**Why the Correct Answer is Right**
The classical triad of symptoms of depression includes:
- **Depressed mood**: a persistent feeling of sadness, emptiness, or hopelessness.
- **Loss of interest or pleasure**: a decrease in activities that were once enjoyable, leading to social withdrawal and isolation.
- **Significant weight loss or gain**: changes in appetite and metabolism, resulting in weight fluctuations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because anxiety is a common comorbidity with depression but not part of the classical triad.
**Option B:** This option is incorrect because insomnia is a common symptom of depression, but it is not part of the classical triad.
**Option C:** This option is incorrect because suicidal ideation is a severe symptom of depression, but it is not part of the classical triad.

**Clinical Pearl / High-Yield Fact**
The classical triad of symptoms of depression is often remembered using the mnemonic "DEPRESSED": D (Depressed mood), E (Energy loss), P (Pessimism), R (Reduced interest), E (Emotional reactivity), S (Sleep disturbances), S (Social withdrawal), E (Energy loss), D (Deterioration in self-care).

**Correct Answer: D. Anxiety.**