Answer
This vitamin c is present due to the process called synthesis.
Step-by-step explanation:
Several molecules that are required by an organism's body are not present in the diet of the particular organism. The organisms hence produce these molecules inside their cells by using other molecules. This process is called synthesis.
As in the above scenario, the laboratory rat does not intake vitamin C in their diet. But as their body requires Vitamin C, so they are capable of producing it in their tissues by using other molecules.