a) C6H12O6 could be seen as CnH2nOn, where n is 6. This can be seen as Cn (H2O)n which makes it seem like carbonx6 plus water (H2O)x6

b) (please see the hardcopy)

c) Yes. The isomers have same chemical formula but different structures, which form different linkages, resulting in different carbohydrates. Different carbohydrates are likely to have different sweetness levels.

There is no such thing as chemical-free food.
Technically, all organic matter are also chemical compounds that can be broken down to their elements. Organic implies natural substances, and chemical would imply man-made substances to a layman.
However, in chemistry, chemicals would also include all matter, including organic food.

A chemical by definition is a molecular compound and everything is a molecule of a kind. There may be different labels between man-made or nature-source, but both are under chemicals.
If something is toxic, it is toxic because of its chemical properties, not because it is a chemical. Sugar from natural sugar canes are fructose, a chemical compound, but still considered organic.