It is true, a circle is named by the center.
A function f is an even function, if for all x we have f(x)=f(-x).
All the functions in the problem are constant functions, because no x is involved.
For example, consider the first function:

,

is just a constant value.( it is actually equal to 0)
Thus for any x, f(x) = 0 and f(-x) =0
This can be said for all constant functions: all of them are even functions.
Answer: all
No.
Consider 10% of 100, which is 10,
and 20% of 10, which is 2