a). |x| = |-x|
This is always true.
The definition of 'absolute' value is 'size of the number without its sign'.
That's what this expression says.
b). |x| = -|x|
This is never true, because an absolute value is never negative.
This one would true if x=0 . So maybe some people might say
it's sometimes true, but that doesn't feel right to me. I say never.
c). |-x| = -|x|
This looks to me like exactly the same situation as (b),
and I would say all the same things about it.