Carcass wrote:
Although no two siblings could have disagreed more in nature—where she was gregarious, he was introverted; where she was outspoken, he was ________—the twins nevertheless got on amazingly well.
A. reserved
B. discreet
C. garrulous
D. insensitive
E. imprudent
F. fluent
The twins got on amazingly well. The word 'Although' suggests us that the blank should work against this.
Discreet and reserved are good antonyms for outspoken, therefore option A & B are right answers.