**Core Concept**
The lungs have multiple functions beyond respiration, including roles in the cardiovascular system, immune response, and endocrine system. The non-respiratory functions of the lungs involve the production and regulation of various substances that impact overall health.

**Why the Correct Answer is Right**
One of the primary non-respiratory functions of the lungs is the production of surfactant, a substance that reduces surface tension in the alveoli, facilitating easier lung expansion during inhalation. The lungs also play a crucial role in the immune system by housing a large number of immune cells, such as macrophages and lymphocytes, which help defend against pathogens. Additionally, the lungs are involved in the production of erythropoietin (EPO), a hormone that stimulates the production of red blood cells in the bone marrow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while the lungs do have a role in the immune system, it is not the only non-respiratory function they possess.
**Option B:** This option is incorrect because the production of surfactant is indeed a non-respiratory function of the lungs, but it is not the only one.
**Option C:** This option is incorrect because the lungs do not play a primary role in the production of insulin.

**Clinical Pearl / High-Yield Fact**
The lungs are also involved in the production of calcitonin, a hormone that helps regulate calcium levels in the blood.

**Correct Answer:** C. The production of erythropoietin (EPO) is a non-respiratory function of the lungs.