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