The proper answer to this question is "kind of sign that is recognized by pretty much everyone".
Reason:
A universal sign would be like a peace sign. Everyone knows what it is and what it stands for (except for children) world peace. Universal signs are used to bring people together for a certain reason. Maybe a problem that was established a certain day of the year (probably a holiday) etc...
Therefore the answer is a sign that everyone knows.
Universal symbols are designed for everyone, no matter where they are globally, to understand something. For example, a universal symbol could be the Red Cross, which symbolises health all over the world
