**Core Concept**
The question is testing the association between environmental factors and mental illness. The underlying concept is the impact of psychosocial stressors on mental health.

**Why the Correct Answer is Right**
Environmental factors, such as living in a broken home, can contribute to increased stress and anxiety levels, which are risk factors for mental illness. This is because a stable family environment provides emotional support and a sense of security, essential for mental well-being. In contrast, a broken home can lead to feelings of insecurity, low self-esteem, and increased stress levels, making it a significant environmental factor associated with mental illness.

**Why Each Wrong Option is Incorrect**
**Option A:** Emotional stress is a symptom of mental illness, not an environmental factor. While emotional stress can contribute to the development of mental illness, it is not an environmental factor itself.
**Option B:** Frustration is a normal emotional response to a situation, but it is not an environmental factor associated with mental illness.
**Option D:** Anxiety is a symptom of mental illness, not an environmental factor. Anxiety can be triggered by various factors, including environmental stressors, but it is not an environmental factor itself.

**Clinical Pearl / High-Yield Fact**
The concept of "social determinants of health" highlights the importance of environmental factors, such as housing, education, and family support, in shaping mental health outcomes. This is a critical area of focus for public health initiatives aimed at reducing mental illness burden.

**Correct Answer:** C. Broken home.