Final answer:
The superior mesenteric vein (SMV) joins the splenic vein to form the hepatic portal vein as part of the hepatic portal system, making D) Splenic vein the correct answer.
Step-by-step explanation:
The question involves the hepatic portal system, which consists of the hepatic portal vein and the veins that drain into it. The hepatic portal vein is formed by the confluence of the superior mesenteric vein (SMV) and the splenic vein. Therefore, the vessel that the SMV joins is the splenic vein. The correct answer to the question is D) Splenic vein.
The other vessels mentioned, such as the inferior mesenteric vein (IMV), left renal vein, right renal vein, and the hepatic vein, serve different functions and are not the correct answer in this context.