Answer:
I'm going to say similar.
Explanation:
Why?
The statement says "ALL circles are to each other. The operative word here is "ALL.'
We can eliminate 'parallel' because that refers to straight lines.
Congruent indicates that they are the same, and ALL circles are not the same in size, diameter, etc...
Concentric means that they have a common center point, and not ALL circles have that.
So, that leaves 'similar.' ALL circles ARE similar; they're ALL round. You can use pi to calculate all of them.