Final answer:
The best method for Steven to determine whether his parents are keeping their promise is direct observation and monitoring of his parents' smoking behavior.
Step-by-step explanation:
The BEST method for Steven to OBJECTIVELY determine whether his parents are keeping their promise would be direct observation and monitoring of his parents' smoking behavior. This method involves Steven personally observing his parents to see if they are smoking or not. This would provide more reliable and accurate information compared to relying on his parents' self-reporting or conducting interviews with their friends and coworkers. By directly observing their behavior, Steven can gather firsthand evidence to determine if his parents are keeping their promise or not.