Hi, thanks for posting!
So in short, our profilers (both memory and performance) work at the process level. You cannot profile just one specific assembly in the sense you describe.
Once you've got your results though, you can use filters to narrow down what you're viewing- in your case, I guess filtering by objects that are in a specific class or namespace may work, assuming that assembly "a" contains a bunch of stuff all within a given namespace. To read about filtering, see here
Hope that helps, but do post back if you have further queries.