Hello,
The answer is option A "stretching".
Reason:
Stretching should be applied before doing any physical activity if not it could cause injuries. Its not option B because yoga involves stretching but it doesn't involve the principles of training. The answer is not option C exercising because thats one of the principles for training. Its also not option D because strength is not involve that should be applied to the principles of training. Which is why the answer is A.
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit