Go to it! My answer will eventually appear in the comments, but right now I suspect the correct response is (D)-- cannot be determined. Why? Because there are four pieces of data we need to know, and we don't know any of them:
1. initial height
2. "diminished" height
3. initial radius
4. "diminished" radius
Still, that's just a conjecture at this point.* When I finally write out my attempted solution, I may disagree with what I've just written. Stay tuned-- but feel free to leave your answer in the comments if you think you've arrived at it before I have.
*If we think of the problem two-dimensionally, i.e., in terms of similar triangles, it may be that there is a solution. Again, I'm typing all this in a rush, so I really don't know yet. Will have to stare hard at the problem before I give it a go.