**Core Concept**
Immunoglobulins, also known as antibodies, are a crucial component of the immune system, providing defense against pathogens and foreign substances. They are a type of protein produced by B cells in response to antigen exposure. Immunoglobulins are secreted into bodily fluids, such as blood and lymph, where they play a vital role in immune function.

**Why the Correct Answer is Right**
The correct answer, 20%, reflects the approximate proportion of immunoglobulins present in the total protein composition of blood plasma. Immunoglobulins are a significant fraction of the proteins in plasma, making up approximately 20% of the total protein mass. This high concentration is necessary for their role in immune defense, allowing them to effectively neutralize pathogens and facilitate the elimination of foreign substances from the body.

**Why Each Wrong Option is Incorrect**
**Option A:** 5% is too low, as immunoglobulins constitute a more significant proportion of plasma proteins. This option is incorrect because it underestimates the importance of immunoglobulins in the immune response.

**Option B:** 10% is still too low, as it does not accurately reflect the concentration of immunoglobulins in plasma. This option is incorrect because it misrepresents the proportion of immunoglobulins in the total protein composition.

**Option C:** 15% is closer to the correct answer but still underestimates the proportion of immunoglobulins in plasma. This option is incorrect because it does not accurately reflect the concentration of immunoglobulins.

**Clinical Pearl / High-Yield Fact**
Immunoglobulins are classified into five main isotypes (IgA, IgD, IgE, IgG, and IgM), each with distinct functions and distribution in the body. Understanding the different types of immunoglobulins is essential for diagnosing and managing various immune-related disorders.

**✓ Correct Answer: D. 20%**