Answer:
cunningly
Explanation:
A synonym for "shrewdly" in this context could be "cunningly". In this case, Ken knew his younger brother convinced their parents to let him go on the adventure, so Ken responded in a cunning manner by answering shrewdly. "Cunningly" suggests that Ken was clever and had an underlying intention to his words, which were meant to protect his younger brother while also asserting his own authority or opinions.
