Assertion: Transgenic mice are being used to test the safety of the polio vaccine.
Reason: It could replace the use of monkeys to test the safety of the vaccine.

The correct option is B Both assertion and reason are correct but reason is not the correct explanation of assertion.
Animals that get their DNA manipulated to possess and express an extra (foreign) gene are known as transgenic animals.

95% of transgenic animals are mice as they can be genetically manipulated in several ways.

Transgenic mice are the most preferred animals nowadays for being used in testing the safety of the polio vaccine.

Transgenic mice are made susceptible to polio virus to ensure the efficacy of the vaccines.

Monkeys have been used for the same purpose, but mice are likely to replace them soon.

Mice are easy to handle in the laboratory, have a considerably short life span, produce numerous offspring after one pregnancy and it is easy to culture the embryos. Hence, mice confer great advantage over monkeys in being used as model organisms.

Thus, both the assertion and reason are independently correct.
However, reason is not the correct explanation of the assertion.