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

The correct answer is C. straightforward.

A. courteous: polite and considerate.
B. correct: free from error or mistake.
C. straightforward: honest and direct.
D. tactful: showing a sensitivity

In this case, the man is being honest and direct, even if it means being blunt. This can be seen as rude to some people, but it is also a sign of his honesty.