The sentence translates to: Today is Thursday. Mañana es translates to: Tomorrow is _. Mañana could also translate to morning, but in this sentence it means tomorrow.
So, with that in mind.. we can go over the answer choices.
Domingo is Sunday.
Sábado is Saturday.
Viernes is Friday.
Miércoles is Wednesday.
The answer is Mañana es viernes.
