Carbon is the only possible solution in this question for the reason that the chemical formula for glucose is c6h12o6 (6, 12 and 6 underscore) and carbon dioxide is co2. Both of the chemical formula contains carbon that is why that is the main element.
The answer is powder because if it was a small crystal it the molecules are tightly compact same with the small cube but there less compact, powder is loose and more spread out and easier to mix so it would react the fastest
