Hi Keshav!
Good question, the MCAT is funny think way.
I think the right way to proceed with a question like this to to make sure you're truly answering the question that that is your primary focus. This question asked 'which experimentation method would be the most effective' and you should proceed, all other considerations aside (incld. ethics), by selecting the option you think would best elucidate what they're looking for. Sometimes, this wouldn't be the right approach - i.e. if you were a physician, you probably wouldn't be making this call. Remember the test is trying to see if you can critically think through the concepts and challenges.
As for a rule of thumb re: ethics, here's my take on it: do not factor ethics into a question unless the question and sometimes the passage asks for it. Med schools will understand your take on these concepts through administering tests like CasPer. For the MCAT focus on logical and critical thought.
Hope this helps!