Answer:
This person is using post-conventional reasoning.
Explanation:
Kohlberg's stages of moral development center around 3 levels:
In this case, the person arrested for stealing the documents states that he/she has a moral obligation to prove that the company is using unsafe chemicals and the public has a right to know about it.
His/her sense of morality goes deeper than any societal relation. The person acted to his/her own accord, deciding to take certain justice into his/her own hands, thus he/she is using post-conventional reasoning.