What are the requirements for obtaining a driver's license according to the given information?
a) Must be at least 16 years old and have held a beginner's permit for 180 days
b) Must be at least 17 years old and have held a beginner's permit for 180 days
c) Must be at least 18 years old and have held a beginner's permit for 90 days
d) Must be at least 16 years old and have held a beginner's permit for 90 days