The correct answer is C. occasionally.
The word “occasionally” means “from time to time; on some but not all occasions.” This is the best fit for the sentence “I have warned him now and then not to resort to violence.” The speaker has warned the person on some occasions, but not all.
The word “consistently” means “in a steady or regular way.” This is not the best fit for the sentence, because the speaker has not warned the person on a steady or regular basis.
The word “frequently” means “happening or done often.” This is not the best fit for the sentence, because the speaker has not warned the person often.
The word “repeatedly” means “again and again.” This is not the best fit for the sentence, because the speaker has not warned the person again and again.