The correct option is D cowardly
Courageous means not deterred by danger or pain; brave. The opposite of courageous is cowardly, or not brave and is too eager to avoid danger. Hence, Option C is correct. Among the rest of the options, frightened means afraid or anxious, naughty means (especially of a child) badly behaved, happy means feeling or showing pleasure or contentment. These options do not express this antonym, hence incorrect.