My best answer for this question would be "ser" because the sentence translated to English is not spoken in future tense. Ser means "be" and estar means "To be". In this case, the question is saying, " You're (blank) very smart." The ser could vary depending on the sentence. It could mean "be" or "are. Basically as long as it is the proper tense for the sentece provided.
I tried my best. Hope this helps you! :)
