<h2>Right answer:</h2><h3>por</h3>
In Spanish, the word por takes on the responsibilities of not only<em> for, </em>but <em>by, on, through, because of, in exchange for, in order to,</em> etc. In this case, the preposition por means <em>because of. </em>Therefore this sentence translates into <em>Because of the weather, it is better to keep the windows open. </em>Finally, our sentence can be written as follows:
<em>Por</em><em> el clima, es mejor mantener las ventanas abiertas.</em>