Hello,
The answer is option D sustains life and keeps body systems in good repair.
Reason:
The answer is D because protein is very important to the body because it fuels your body without protein you wouldn't be able to live. Its not option A because protein does not help you build muscles. Its not option B it doesn't keep your nails or your hair. Its also not C because protein gives energy not collagen.
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit