Answer:
B. Don John's hatred of his brother, who defeated him and reigns as the prince.
Explanation: One reason is that Don John blames Claudio for preventing him from overthrowing his brother. Apparently, when Don John tried to overturn his brother's rule, Claudio, as Don Pedro's right-hand man, devised some clever tactics to prevent the overthrow and win this civil war for Don Pedro.
The illegitimate brother of Don Pedro, Don John is melancholy and sullen by nature, and he creates a dark scheme to ruin the happiness of Hero and Claudio. He is the villain of the play; his evil actions are motivated by his envy of his brother's social authority.
