Final answer:
The correct verb to fill in the blank is "estaba".
Step-by-step explanation:
The correct verb to fill in the blank in the sentence is estaba. The sentence is saying "Carlos was studying in the library while I was at the concert." In this case, we use the verb "estar" (to be) in the past tense to describe the action of studying. The appropriate form of the verb is "estaba" to match the subject "Carlos."
Learn more about Spanish verb conjugation