c. words are not firm and dependable

These words are actually a literary device called a metaphor.    Metaphors are basically comparisons of things that are unlike.  Metaphors are used, mainly, to give readers a sense of understanding when there may not otherwise be one.  What this means is that authors will compare something that may not be known to readers to something that most likely will be known in order to present the best image understood by the most readers.  Everyone will know what words (promises) are, and everyone will know what wind is.  But, to compare words/promises to wind means words/promises will take on the symbolic meaning of what wind is like.  For instance, wind can be felt, but one cannot hold it, per se--it is not firm.  Also, the blowing and direction of the wind is always uncertain, and it can never be counted on to continue blowing in or from a certain direction.  Thus, the words  “words are but wind” mean that promises really mean nothing because one cannot trust promises because they can be broken and people (the promisors) are flippant and are prone to changing their minds just like the wind changes direction.