Final answer:
To obtain a 7D certificate, one must be at least 18 years old. Additional requirements may apply such as training and background checks.
Step-by-step explanation:
In order to obtain a 7D certificate, which is typically associated with the authorization to operate school buses or transport students, one must meet certain age requirements among other qualifications. The correct answer to the question is a) 18 years old. Regulations can vary by state or country, so it's important to check with the local motor vehicle department or the relevant governing body for specific requirements in your area. However, at a minimum, a person looking to acquire a 7D certificate needs to be at least 18 years old. Additional requirements might include passing a background check, completing a training program, obtaining a commercial driver's license (CDL), and passing a physical examination.