Leading factors of ecological migration

① The immigrants in the Three Gorges reservoir area are first of all the needs of engineering construction, and do not belong to ecological immigrants, which does not conform to the meaning of the question;

② Peasants in western Ningxia moved out mainly because of the arid climate, belonging to ecological migration, so it was correct;

(3) The educated youth in the east moved to the western frontier, because it was a political factor, so it did not conform to the meaning of the question;

④ It is correct for farmers in southern Shaanxi to move out of the affected mountainous areas because of ecological problems.

Therefore, choose: d.