Ahmad writes, "I am a GC and I often travel to Muslim countries, most of them are hostile to the United States. Will I have any problem when I file for naturalization?"
Everyone, when they return to the United States, are subject to questioning by a border agent. While permanent residents and citizens will definitely be allowed to enter, nonimmigrants maybe even turned away if the officer suspects that their entry into the United States is a threat to national security. If you have done nothing wrong and are visiting for leisure/family reasons/business, while there maybe more questioning at the time of returning, you have nothing to fear. When you file for citizenship, it would not be surprising that your background check is very thorough and takes a long time, but if you have nothing criminal in your background, you will be approved. Unfortunately, individuals whose names are similar to terrorists and criminals can get stuck for years, particularly if they also have travel frequently to American enemy nations.