Usage patterns
正当な〈事〉a legitimate/justifiable ~ (reason, claim, etc.)
He had a legitimate reason for quitting the company.
〈事〉を正当と認めるto acknowledge something as legitimate/justified
The court recognized the claim as legitimate.
正当な権利a legitimate/rightful claim (right)
That is her rightful entitlement.
正当に〈動詞〉to (do something) fairly/properly
His efforts should be fairly evaluated.