The correct answer is 'bitten'. As the given sentence is in passive form i.e. the person or thing doing the action (the agent) is usually preceded by the word "by" and the person or the thing on which the action is being performed is at the beginning of the sentence, then the structure of the sentence would be :Object +Helping Verb + V3+By+ Subject.Thus, if we go by the rule and structure of the sentence then v3 form of bite i.e. bitten should be the appropriate option to be chosen for filling the blank.Hence, the correct answer is option 3. The sentence would be: The old beggar was bitten by a mad dog.