To solve this problem, we need to identify which structure prevents the backflow of undigested and unabsorbed substances into the small intestine once they enter the caecum.$\newline$Let's analyze each option:$\newline \bullet$Sphincter of Oddi:$\newline$ - This is a muscular valve that controls the flow of digestive juices (bile and pancreatic juice) from the liver and pancreas into the duodenum.$\newline$ - It is not involved in preventing backflow from the caecum.$\newline \bullet$Ileo-caecal valve:$\newline$ - This is a sphincter muscle situated at the junction of the ileum (last part of the small intestine) and the caecum (first part of the large intestine).$\newline$ - It prevents the backflow of contents from the caecum into the ileum.$\newline$ - This is the correct structure that prevents backflow.$\newline \bullet$Gastro-oesophageal sphincter:$\newline$ - This is a valve located between the esophagus and the stomach, preventing the backflow of stomach contents into the esophagus.$\newline$ - It is not related to the caecum.$\newline \bullet$Pyloric sphincter:$\newline$ - This is a valve that controls the passage of food from the stomach to the duodenum.$\newline$ - It does not prevent backflow from the caecum.$\newline$Therefore, the correct option is Option 2: Ileo-caecal valve.