Answer:
I think the first one is the best answer but you might be expected to say variable. I cannot choose one answer. It all depends what you have been told or what your teacher has said about it.
Myself, I would pick A.
Explanation:
Symbols (like letters) that are used to represent numbers which are unspecified.
The problem is that variable is also a possible answer. The problem is that a variable is usually a letter. Symbols do are not used for variables in anything but counting systems.
When you look up the word variable it is defined as a symbol. But that is too confining.