The correct answer is: Only argument I is strong.
Argument I is strong because it provides evidence that rejecting vehicles older than 15 years will lower the pollution level in metros. This is a valid argument because it is based on the assumption that older vehicles emit more pollutants than newer vehicles.
Argument II is weak because it does not provide evidence that rejecting vehicles older than 15 years will make it difficult for vehicle owners to shift to other parts of the country. This is an invalid argument because it is based on the assumption that all vehicle owners live in metros and that they will not be able to find suitable jobs in other parts of the country.
In conclusion, only argument I is strong.