The Answer

The greatest percentage of cases of superior vena cava syndrome is caused by C. Non-small cell lung cancer.

The greatest percentage of superior vena cava syndrome cases are caused (in descending order) by non-small cell lung cancer, small cell lung cancer, and non-Hodgkin lymphoma. Superior vena cava syndrome is often the presenting condition which leads to those cancer diagnoses. 

Less common causes include thyroid cancer, melanoma, Kaposi sarcoma, and tuberculosis.