The answer is (3).
“would help” is conditional present whereas “would have helped” is conditional perfect.
Firstly, both of them are used for hypothetical scenarios that did not happen, or could never happen, either because the event has passed or it’s just impossible.
“would help” is used to talk about something that is a general truth or something that could happen in the future.
“would have helped” is used to talk about what the subject would have done if he was able to go back in time to a particular event.
In this context, the writer seems to be expressing a general truth that should the old lady need help, either now or in the future, he would definitely help her out. This is why the answer is 3.
If you were to use “would have helped” a past event or time needs to be referenced.
So correct usage of “would have helped” could be:
- I would have helped the old lady if I knew she needed help yesterday.
- I would have helped the old lady at the supermarket just now if I knew she was having trouble.
- I would have helped the old lady if I knew she needed help then.