To understand them, you have to specialize yourself in them. I got my MSc in Physics 2.5 years ago and am currently working on my PhD, both in experimental solid-state physics. And guess what - I haven't really understood those theories. Basic QM
questions, ok, but quantum field theory, string theory, all that stuff, forget it without in-depth lectures and loads of reading.
WARNING! The post above should be taken with a grain of salt! Furthermore, it may be slightly biased towards A.