This is present in a rat and not in a frog

The correct option is jejunum.

To understand why this is correct, we can compare the anatomical structure of the digestive systems of mammals (like the rat) and amphibians (like the frog):

1. Small Intestine of the Rat (Mammal):
In rats, the small intestine is long and highly differentiated into three distinct parts:
- Duodenum: The short, C-shaped first loop of the small intestine.
- Jejunum: The highly coiled middle section where the majority of nutrient absorption takes place.
- Ileum: The final portion that connects to the large intestine.

2. Small Intestine of the Frog (Amphibian):
In frogs, the digestive system is less complex. The small intestine is shorter and consists of only two main regions:
- Duodenum: The anterior part running parallel to the stomach.
- Ileum: The coiled posterior part that opens directly into the rectum. Frogs do not have a distinct middle section corresponding to the jejunum.

3. Other options:
- Both rats and frogs have a stomach for preliminary food storage and digestion.
- Both rats and frogs have a duodenum and an ileum as parts of their small intestines.

Therefore, the jejunum is the structure that is present in the rat but absent in the frog.