Final answer:
Accession is the improvement to property through labor that increases its value, which in this case applies to the customizing of a 1967 Ford Mustang. Option a.
Step-by-step explanation:
The result of Mike's efforts in customizing and detailing the 1967 Ford Mustang to Lauren's specifications, which led to the car earning several awards at regional vehicle customizing competition shows, is best described as an accession option a. Accession in this context refers to the addition or improvement to personal property through labor, resulting in an increase in value.