In the chemical reaction in which sucrose is heated and decomposes to form carbon dioxide and water, which of them is a reactant?

The reactant here is sucrose. It is the substance that decompose into carbon dioxide and water. Sucrose is the one who undergoes a chemical change in the process therefore it is the reactant. Carbon dioxide and water on the other hand are the products of the reaction.

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"