All of the answers besides "B" are ambiguous at best.
"Against" her boss's decision could be translated to "in spite of". If she was angry "in spite of" her boss's decision, that means she would be angry even though she agrees with her boss - which makes no sense, in this case.
"By" her boss's decision is in the wrong tense. If the sentence was "Fiona was angered __ her boss's decision...", then it would have made sense to use "by".
"For" could be translated to "in place of", which, again, makes no sense given the context.
"B" appears to make most sense, considering "about" can be translated to "concerning" or "with regards to", which sounds much more elegant than the other options.
Hope this helps.