Abundant and powdery pollen produced by small, indistinct flowers is probably transported by:
bees and butterflies
flies
birds
wind

Wind

Explanation:

Commonly, a flower that uses wind as dispersion, tend to be:

Small

White of light colors

Produces a lot of pollen

Inconspicuous or indistinct

The reason behan the reason behind that is that they don't need to attract attention, they just need the wind to get rid of the flower and thus achieve propagation.

substitution is the answer in this case