Blue, long horn, long touch: it is seen with the eyes; Gu Ming: I heard it with my ears. So it belongs to the observation method.
B the conversation method is wrong. Because people and locusts can't communicate in words.
C the investigation method is also wrong. Because I didn't say I would investigate many different locusts.
D the experimental method is also wrong. Because that sentence didn't say how to design and do the experiment.