That a Male European Robin Will Attack a Tuft of Red

Multiple Choice

That a male European robin will attack a tuft of red feathers as readily as he will another male robin indicates that

A) an attack will be produced by almost anything.
B) intruding males and females will be attacked equally often.
C) stimulation of attack behavior requires a complex combination of stimuli.
D) the red feathers serve as a sign stimulus for attack behavior.