B, according to the material analysis in the topic: the river sand at the foot of the mountain is a landform formed by flowing water deposition; Being blown by strong winds year after year is the carrying function of the wind; A huge abrupt sand dune was formed on the river bank, indicating that it was formed by wind. Therefore, the main external force process of "Fozhang Dune" is: flowing water deposition-wind transport-wind deposition, which is correct;
C, sand dunes are formed by wind deposition, so they do not meet the meaning of the question;
D. "Fozhang Dune" is located at the foot of the northern bank of the Yarlung Zangbo River. In the dry season, river sand often appears in nearby rivers, which is blown away by strong winds year after year and gradually forms a huge and abrupt sand dune on the river bank. According to the analysis of this sentence, the river sand at the foot of the mountain is a landform formed by flowing water deposition; Being blown by strong winds year after year is the carrying function of the wind; Huge and abrupt sand dunes have formed on the river bank, which shows that it is formed by wind, which does not meet the problem.
Therefore, choose: B.