**Core Concept**
Illingworth's theory describes the formation of a specific type of salivary gland tumor, which involves the abnormal presence of ectopic parotid epithelium in the upper deep cervical lymph nodes. This phenomenon is a result of aberrant embryological development, where parotid gland cells migrate to lymph nodes during fetal development.

**Why the Correct Answer is Right**
According to Illingworth's theory, the inclusion of ectopic parotid epithelium in the upper deep cervical lymph nodes leads to the formation of Warthin's tumor, also known as adenolymphoma. Warthin's tumor is a benign salivary gland neoplasm that is characterized by the presence of lymphoid stroma and oncocytes. The ectopic parotid epithelium in the lymph nodes differentiates into oncocytes, which are a type of epithelial cell that exhibits oncogenic properties.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the type of tumor formed by the inclusion of ectopic parotid epithelium in the upper deep cervical lymph nodes.

**Option B:** This option is incorrect because it is a different type of salivary gland tumor that is not associated with Illingworth's theory.

**Option C:** This option is incorrect because it is a different type of neoplasm that is not related to the presence of ectopic parotid epithelium in lymph nodes.

**Clinical Pearl / High-Yield Fact**
Warthin's tumor is a benign salivary gland neoplasm that is often associated with smoking and is typically seen in elderly individuals. It is characterized by the presence of lymphoid stroma and oncocytes, and is easily distinguishable from other types of salivary gland tumors.

**Correct Answer:** D.