Combined with the meaning of the question, "it is found that the shadow of the flagpole is shorter every day, and the length of the shadow is obviously shorter than the length of the flagpole", which shows that the height angle of the sun is getting bigger and bigger, and the shadow will become shorter and shorter.
Combined with the chart, the earth should move from the equator to the Tropic of Cancer during the operation from A to B.
So choose: a.