I would personally define internal as it relates to what I learned from/of Chinese internal as:
The Ideally most efficient use of the body as a tool. Relating especially to physical structure and alignment.
Would you say this is something relating to "C" or something similar or something different all together? How and/or why?
I can see how "C" might be an important part of learning "D" but not a necessity. Could you briefly explain a bit more?
I think it's a way of distinguishing between internal and external training that internal training enables you to be stable and powerful even if your physical structure and alignment are compromised. It makes it harder for them to control you and easier for you to control them. I think that's "C".
At a more advanced level they won't even understand what's happening. I think that's "D".