**Core Concept**
The spread of cancer cells from the primary site to distant locations in the body is known as metastasis. Metastasis occurs through a complex process involving invasion, intravasation, circulation, extravasation, and colonization. The most common sites of metastasis depend on the primary tumor's origin and the patient's overall health.

**Why the Correct Answer is Right**
Lungs are the most common site of metastasis due to their high vascularity and the fact that many cancers, such as breast, colon, and melanoma, initially spread to the lungs via the bloodstream. This is because the lungs are the first capillary bed that cancer cells encounter after entering the circulation. The high oxygen tension in the lungs also promotes the growth and proliferation of metastatic cells. Additionally, the lungs' ability to filter the blood and remove particulate matter makes them an ideal site for cancer cells to lodge and establish a new tumor.

**Why Each Wrong Option is Incorrect**
**Option A:** Liver is a common site of metastasis, especially for gastrointestinal cancers, but it is not the most common site overall.
**Option B:** Bones are a common site of metastasis for cancers such as breast, prostate, and lung, but they are not the most frequent site.
**Option C:** Brain metastasis is more common in certain cancers, such as melanoma and lung cancer, but it is not the most frequent site of metastasis overall.

**Clinical Pearl / High-Yield Fact**
Remember the "Rule of 3s": 3% of breast cancers metastasize to the brain, 3% of lung cancers metastasize to the adrenal glands, and 3% of gastrointestinal cancers metastasize to the ovaries.

**Correct Answer:** C. Lungs