Final answer:
The Doppler waveform obtained during an abdominal Doppler ultrasound study is most likely sampling the Portal vein (D).
Step-by-step explanation:
The Doppler waveform obtained during an abdominal Doppler ultrasound study is most likely sampling the Portal vein (D). The hepatic portal system is responsible for delivering blood from the digestive and circulatory organs directly to the liver for processing. The portal vein is a major vessel in this system that carries partially deoxygenated blood containing nutrients absorbed from the small intestine. It enters the liver and supplies oxygen and nutrients to the liver tissues.