Same two characters as 正当, the time word meaning 'just when', but the second syllable is fourth tone here. If the sentence is judging whether something is proper or lawful, the reading is 正当.
usage
It mostly turns up in set pairs: 正当理由 a legitimate reason, 正当权益 lawful rights, 正当竞争 fair competition. The negative 不正当 is just as common and is the usual way to call something improper.
