Clever or bright—having quick understanding and mental agility; used in a somewhat literary or formal sense.
彼女は利発な子どもで、学校の先生からも褒められている。
She is a bright child and is praised by her teachers.
会議での質問に的確に答える利発さが彼の長所だ。
His ability to answer questions accurately in meetings is his strength.
利発という言葉はやや古風だが、意味は『頭が良くて反応が速い』ということだ。
The word 'rihatsu' is a bit old-fashioned, but it means 'quick-witted and intelligent.'