Decent little bangers individually but as a mob just pitiless and implacable ..what implacable mean here?

Adamant; immovable.

http://www.wordnik.com/words/implacable

having or expressing very angry or determined feelings that will not change

http://www.macmillandictionary.com/dictionary/amer...

Relentless; unstoppable.

https://en.oxforddictionaries.com/definition/us/im...

not controllable, not able to be satisfied or calmed. wild, but only in the unmanageable sense of the word. I would accept either c or d. not sure what the asker would accept. probably only d. It definitely is not wild in the sense of savage despite the association with pitiless.

Not what I call a "good" question.