Answer: para siempre
Explanation: The sentence can be translated to:
"With the new environmental law that we will have, the problem of throwing trash will not exist ___"
And the options are:
Por escrito (For written)
Para siempre (forever)
está para (this for)
Para entonces (then)
The correct option is Para siempre, which is not "perfect", but is the one that has the most coherence with the sentence.
Con la nueva ley de medio ambiente que tendremos, el problema de tirar basura no existirá para siempre.