The correct option is A. Winner.
An object complement is a noun, pronoun or adjective that follows a direct object to explain in what it has become. Certain verbs "of making" such as consider, name, catch and find are likely to have object complements. A direct object is a noun or phrase that contains the receiver of the action of a verb. In this sentence, the "receiver" of the action to "judge" is the "entry". And that entry became the "winner". As the word "winner" changes the state of the entry and is after a direct object, it is the objective complement of the sentence.