noun

a call to someone to participate in a competitive situation or fight to decide who is superior in terms of ability or strength.

he accepted the challenge

dare,provocation,summons

verb

dispute the truth or validity of.

it is possible to challenge the report’s assumptions

question,disagree with,object to,take exception to,confront,dispute,take issue with,protest against,call into question,demur about/against,dissent from,be a dissenter from