Step-by-step explanation:
The substance which is white in color is heated which give further different products which means that A is composed of different elements and thus A is a combination of elements and is a compound.
The product obtained when carbon is burnt in excess of air is carbon dioxide gas. Since, it is mentioned in the question that gas C has exactly the same properties as carbon dioxide which also suggests that C is also definitely a compound.
It is not very sure about B, whether it is a compound or a element as there is not much information available for B. But, B is likely to be a compound because a very few elements have white color but still it's not sure.