intelligent,
clever,
alert,
quick-witted mean mentally keen or quick.
intelligent stresses success in coping with new situations and solving problems
<an intelligent person could assemble it fast>.
clever implies native ability or aptness and sometimes suggests a lack of more substantial qualities
<clever with words>.
alert stresses quickness in perceiving and understanding
<alert to new technology>.
quick-witted implies promptness in finding answers in debate or in devising expedients in moments of danger or challenge
<no match for his quick-witted opponent>.