What should you do if you are unable to find the car registration documents?

Answer: C — Request a replacement from the law enforcement officer

If you are unable to find the car registration documents, contacting the Department of Motor Vehicles (option A) is not necessary as they will not be able to provide you with a replacement. Additionally, option B suggests contacting the registered owner, but this may not be possible if you are not the registered owner or do not have their contact information. Option D is not a reasonable or safe course of action. Therefore, the best course of action would be to request a replacement from a law enforcement officer (option C), as they have the authority to provide you with a replacement or guide you through the proper steps to obtain one.