The differentiating factor of the venous system of frog and rabbit is in the presence of this

The correct option is renal portal system.

To understand why this is the differentiating factor, let's break down the anatomy of the venous systems of both the frog (an amphibian) and the rabbit (a mammal) step-by-step:

1. Understanding Portal Systems:
A portal system is a part of the circulatory system where blood passes through two capillary beds before returning to the heart. There are two major portal systems in vertebrates:
- Hepatic Portal System: Carries blood from the digestive tract to the liver. This system is present in both frogs and rabbits.
- Renal Portal System: Carries blood from the posterior parts of the body (like the hind limbs and tail) to the kidneys. The kidneys filter this blood before it goes back to the heart.

2. Comparing Frog and Rabbit Venous Systems:
- Frog (Amphibian): The venous system of the frog includes both a well-developed hepatic portal system and a prominent renal portal system. Blood from the hind limbs is collected by the renal portal veins and routed through the kidneys.
- Rabbit (Mammal): Mammals, including rabbits, have completely lost the renal portal system during evolutionary development. Instead, blood from the hind parts of the rabbit's body flows directly into the postcaval vein (inferior vena cava) without first entering a capillary network in the kidneys.

3. Looking at the Other Options:
- Hepatic vein: Present in both frogs and rabbits to drain blood from the liver into the main venous system.
- Hepatic portal system: Present in both frogs and rabbits to transport nutrient-rich blood from the digestive tract to the liver.
- Three vena cavae: Both frogs and rabbits possess two anterior vena cavae (precavals) and one posterior vena cava (postcaval) entering the heart (or sinus venosus in frogs), so this is not the unique differentiating factor.

Therefore, the presence of the renal portal system in the frog and its complete absence in the rabbit is the primary differentiating factor between their venous systems.