THE SOLUTION:
He chose the Crossfire. Cal’s dad would only buy him a vehicle whose name and make began with the same letter (a Chevy Camaro, a Toyota Tundra, a Mazda Miata, a Ford Focus) but not different letters (a Ford Mustang, a Dodge Ram, a Jeep Cherokee, a Honda Civic). Calvin figured this out, and correctly chose the Chrysler Crossfire over the Lincoln Continental.
Did you solve the puzzle? Or did you come up with a different reason and/or a different car? Feel free to share your ideas (kindly, please) in the comments. Thanks!