In frog, the Renal portal system is a special venous connection that acts to link :

The correct option is Kidney and lower part of body.

In amphibians like frogs, the venous system exhibits unique vascular arrangements called portal systems. A portal system is a specialized system of veins that begins and ends in capillaries, carrying blood from one organ to another before returning it to the heart.

The Renal Portal System specifically serves to connect the lower parts of the body (such as the hind limbs and tail region) to the kidneys.
Instead of returning directly to the heart, venous blood from the lower part of the body is collected by the renal portal veins. This blood is routed to the kidneys first, where waste products are filtered out before the blood enters the postcaval vein to return to the heart.

Let us briefly distinguish this from other options:
- The connection between the liver and intestine is known as the Hepatic Portal System.
- There is no direct portal link termed "liver and kidney" or "kidney and intestine" in the frog's circulatory anatomy.

Therefore, the renal portal system specifically links the kidney and the lower parts of the body.