The correct option is C ashamed of
Proud means having or showing a high or excessively high opinion of oneself or one's importance. The opposite of proud is ashamed, or embarrassed or guilty because of one's actions, characteristics, or associations. Hence, Option C is correct. Among the rest of the options, unsure means not feeling, showing, or done with confidence and certainty, worried means anxious or troubled about actual or potential problems, concerned means worried, troubled, or anxious. These options do not express this antonym, hence incorrect.