There are some compounds in which the electrons move from one side to anothe, and it is a continous movement. For that reason, the resonance structure is created. It works as a theorical structure which is not exact, but it helps us to approach to the real structure.
For example, for the bencene, there are 3 different resonance structures:
And they are the same compound. It means that the answer is B. Alternate structures of the same compound.
