challenge

[1]	question , dispute , defy , object to , take exception to , contest , doubt , call into doubt , call into/to question , impugn
[2]	invite , dare , summon , call out , provoke
[3]	brave , dare , confront , defy , contest
[4]	gainsay
[5]	take exception

[1]	question , dispute , doubt
[2]	invitation , dare , summons , provocation , confrontation , defiance , ultimatum
[3]	problem , demand , stimulation , trial , test