## Core Concept
The diagnosis of depression, particularly in the context of daily manifestation, relies on the duration and frequency of depressive symptoms. According to the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) and the International Classification of Diseases (ICD-11), depression is diagnosed based on the presence of specific symptoms over a defined period.

## Why the Correct Answer is Right
The correct answer, , indicates that at least 2 weeks is required to diagnose depression when symptoms are present daily. This duration criterion is crucial for distinguishing transient depressive reactions from a clinically significant depressive episode. The DSM-5 specifies that for a diagnosis of major depressive disorder, at least five of the following symptoms must be present during the same 2-week period, with at least one of the symptoms being either depressed mood or loss of interest or pleasure. The symptoms must cause significant distress or impairment in social, occupational, or other areas of functioning.

## Why Each Wrong Option is Incorrect
* **Option A:** is too short a duration for a diagnosis of depression with daily symptoms. Such a brief period does not adequately distinguish between transient mood disturbances and a clinically significant depressive episode.
* **Option B:** might seem reasonable but still falls short of the required duration for diagnosing depression with daily manifestation.
* **Option D:** exceeds the minimum required duration and, while prolonged periods of symptoms can indicate more severe or persistent depression, is not the minimum threshold.

## Clinical Pearl / High-Yield Fact
A key point to remember is that for a diagnosis of major depressive disorder, the symptoms must be present for at least 2 weeks. This criterion helps in ensuring that the condition is chronic enough to be considered a disorder rather than a transient reaction. A useful mnemonic is not directly applicable here, but recalling that "two weeks" is the benchmark can help in making accurate diagnoses.

**Correct Answer: C. 2 weeks.**