His forthright behaviour shows that he is honest but he seems rude to some people.

Answer: Option C

Courteous : polite, respectful, or considerate in manner.
Correct : free from error; in accordance with fact or truth.
Straightforward : (of a person) honest and frank.
Tactful : having or showing skill and sensitivity in dealing with others or with difficult issues.
Forthright : (of a person or their manner or speech) direct and outspoken.

Synonym of Forthright is Straightforward