To say that symbols are arbitrary means a. they have several possible meanings.
b. they have no direct relationship to the things they represent
c. they are not material or physical.
d. all of the above.

Explanation:

It is really D: All of the above.

Usually an arbitrary symbol is thought of as B. And that could very well be the answer. They don't have any seeming relationship to the object they represent.

But it is also possible that they need not be material or physical.  The contents of a letter can be an arbitrary symbol.

Because it is arbitrary, it can have several meanings. The symbols in Revelation in the Bible certainly fall into this category, although many Christians would dispute this point.

I'd pick D, but B is certainly a possible choice.