**Core Concept**
A metastatic brain carcinoma in an adult most often originates from a primary tumor in another part of the body, typically through hematogenous spread. This phenomenon is often associated with certain types of cancers that have a high propensity to metastasize to the brain.

**Why the Correct Answer is Right**
The primary site of metastatic brain carcinoma in adults is most commonly the lung, especially in patients with a history of smoking or other forms of lung cancer. This is due to the high incidence of lung cancer, the tendency of lung cancer cells to break away from the primary tumor and enter the bloodstream, and the brain's rich blood supply, which facilitates the lodging of tumor cells. The brain's blood-brain barrier can sometimes hinder the spread of metastases, but lung cancer cells have developed mechanisms to bypass this barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** Stomach cancer is less likely to metastasize to the brain compared to lung cancer, although it can occur in rare cases. The primary site of metastasis for stomach cancer is more commonly the liver and peritoneal cavity.
**Option B:** Ovarian cancer can metastasize to the brain, but this is relatively rare and typically occurs in the context of advanced disease. The primary site of metastasis for ovarian cancer is more commonly the peritoneal cavity and lymph nodes.
**Option C:** Oral cavity cancers can metastasize to the brain, but this is also relatively rare and typically occurs in the context of advanced disease. The primary site of metastasis for oral cavity cancers is more commonly the regional lymph nodes and neck.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the most common primary sites for metastatic brain tumors in adults are the lung, breast, and melanoma, in that order. This knowledge can aid in the differential diagnosis of brain metastases and guide further investigation and management.

**✓ Correct Answer: D. Lung**