The is the protective barrier established by the police by which they protect one another from outsiders, often even refusing to aid police superiors or other law enforcement officials in investigating wrongdoing of other officers.

Explanation:

It means the informal rules among the police officers  due to which they do not report, police brutality, crimes or misconducts  is called the blue wall of silence.  If a police officer is being questioned about the misconduct of his colleague, and if the police officer is following this code would not reveal anything about  his colleague's wrong doing.

Hence, in this scenario blue wall of silence is the correct answer.

answer; eliminate parking problems in cities, because a car took up only half as much space as a horse and buggy;