Tough choice between C and D
Will go with C (though D still stares at me
In the original sentence:the idea of penitentiaries were reforms of the penal system by Quakers.
means that penite...s introduced by Quakers reformed the penal system
My guess is C comes closest to this and Do would have been correct if :Quakers suggested that the penal system be reformed by penitentiaries
I'm not sure about that last sentence. That means that penitentiaries [as entities capable of performing an action] will be reforming the penal system?
If you meant "replaced by" then I don't think "reformed by" is proper proxy.
Ex: Bill 120 will be reformed by Bill 199 --> wrong
Bill 120 will be reformed by the House of Common --> right context. "reformed by" requires an entity to perform the action of reforming