Final answer:
Kara's challenges in learning a third language are most likely due to language interference.
Step-by-step explanation:
The challenges that Kara, a bilingual adult, is facing in learning a third language are most likely due to language interference. Language interference occurs when the knowledge and structures of one language have an effect on the learning and use of another language. In Kara's case, her previous knowledge of two languages may be influencing her attempt to learn a third language.