Answer:
C: March 10
Explanation:
In this scenario he signed the security agreement on the 5th of March, picked up the car on the 10th of March, and filed the security agreement on the 15th of March.
Even though he signed the agreement on the 5th of March (which would be believed is the day that the security interest is attached), he only took it into his possession on the 10th. Only once you take the car into your possession does the security interest attach, since it is no longer in the possession of the car dealer.