D, all of the above. This is a goal dependent question and answer.
There are a couple of principles of training that we should keep in mind when considering this topic; Specificity and Specific Adaptations to Imposed Demands (S.A.I.D). With those principles in mind lets take a look at this topic.
Is muscle isolation a good practice? Sure if you’re a body builder, there is not doubt in any professionals mind that training in isolation works for the training of a specific muscle or group. Is this optimal? Sure if you’re a bodybuilder and don’t care about performance. I hope you are seeing a trend here. Bodybuilding is most definitely in favor of isolation type of movements..
Now lets get to the other side of this story…Is muscle isolation a good practice? No, not if you are an athlete. There is a reason that the top level athletes in the world don’t do Isolation exercises…they decrease intra-muscular coordination, and in doing so limit performance increases. Is this optimal? I am sure you can guess my answer to this one..NO, it is not optimal for athletic performance increases.
There is however another topic her for Isolation exercises and that covers physical therapy and corrective exercise strategies.In the realm of Therapy and Corrective strategies there is most definitely a use for isolation type of exercise.
There seems to be much debate here whether or not you should train isolation exercise….the facts are simple and clear. If your goal is not dependent upon increased function or athletic performance then by all means train that way, but if you want to move like well balanced fine tuned human machine then limit the amount of isolation exercises.