Of the choices given, gang crimes pose by far the greatest threat to law enforcement. This is mainly because such crimes are much more common than the other two types of crime mentioned.
If ritualistic crimes and, particularly, hate crimes were more widespread, they would be a more important threat. Hate crimes would be a particular threat because our diverse society could be badly harmed by such crimes. If we were to have many crimes being committed against people because of their race or religion, it could pull our society apart. Luckily, these crimes are not particularly common and therefore they are not a grave threat.
By contrast, gang crimes are very common and they do pose an important threat to the communities in which they occur. There are many poorer areas in cities where gangs dominate life. In such communities, it can be dangerous to do something as simple as walking to school. This drastically reduces the quality of life in the community and it also makes it much harder for young people from such neighborhoods to get ahead in life. This makes gang crimes a major threat to some parts of our society and, therefore, to law enforcement.