"They know each other."
Translation:그들은 서로 압니다.
그들은 = They (topic)
서로 = reciprocally
알다 "to know" formal conjugation, drop the ㄹ -> 압니다.
If I didn't know who they are, I should assume they are boys using 그들? Or even if it was one boy and one girl?
The answer choice for 그들은 is typed differently and I don't know how to report for that.