The ability to sing one's national anthem, such as "the star-spangled banner," without much difficulty would most likely be a result of:

The result would be most likely of recognition. It is because if a person has encountered the song because it is their national anthem, she or he would most likely recognize it and would able to remember it. This will give the result of the person singing their national anthem without difficulty because he or she already knows about it and recognizes it.