The correct answer is 'what the shortest way back was'. The given sentence is in the form of indirect speech. It focuses more on the content of what someone said rather than their exact words. In indirect speech, the structure of the reported clause depends on whether the speaker is reporting a statement, a question or a command.For example:- Who are you? she asked.(Direct speech) -> She asked me who I was.(Indirect speech)When we write a sentence in indirect speech in which the speaker is reporting a question, then we dont use a question mark and the reported clause is in statement form (subject + verb), not question form.Thus, if we go by the rule and structure of the sentence in indirect speech for questions, then the correct answer would be option 3. The sentence would be: He asked me what the shortest way back was.