You might like to try and see if there are any clues that you can glean from the text itself before you answer this question. Often, locating the word or phrase you are trying to find the definition for and then reading around it can yield interesting and valuable contextual clues that you can use to establish a definition. Let us try this process in this case:
He sprang to his sleigh, to his team gave a whistle,
And away they all flew like the down of a thistle.
Note that "down" is part of a simile, where the speed and lightness of the sleigh is compared to the "down of a thistle." Trying to eliminate some of the answers that you have, you can definitely erase "destitute," as that has no relevance. Likewise, "not up" contradicts the idea of the sleigh rising with ease. "Padding" likewise makes no sense, leaving you with "soft, short fibres" that refer to the "down" of the thistle, and are very light and rise into the air very easily with the slightest gust of breeze.