Final answer:
Traffic law commonly requires that vehicles park at least 50 feet away from the nearest rail of a railroad crossing, which is not among the options provided in the question.
Step-by-step explanation:
The question about how close to a railroad crossing one is permitted to park is a matter of traffic law, which can differ by jurisdiction. Based on general traffic safety rules, it's commonly required that vehicles park at least 50 feet away from the nearest rail of a railroad crossing to ensure safety and visibility. This would typically be much more than the options provided (5 feet, 10 feet, 15 feet, 20 feet), indicating that none of the given choices are correct unless there is a specific local ordinance that states otherwise.