The first thing we are going to do for this case is to translate the incomplete sentence into English.
We have then:
The __________ is where you sit in the cinema or in the theater.
We now choose a word to complete the sentence.
We have then:
Chair
Substituting we have:
The chair is where you sit in the cinema or in the theater.
The translation of the sentence in Spanish is:
La silla es donde te sientas en el cine o en el teatro.
Answer:
Silla
La silla es donde te sientas en el cine o en el teatro.