Answer: C "why"
I believe it's C "why"
If you are persuading someone, the thing you want to tell people is the good things you are doing it for. Or the "why" you are doing this.
For example,
Can we adopt a dog?
This does not say the why or the reason for it so you are not persuading as much as you could in this sentence.
If you said,
Can we adopt a dog to give it a better home than it had and the care that it needs?
This sounds more persuasive because I am stating the reason for it or the "Why" of the sentence.