When sugar is heated it breaks down into Carbon, Water and C.arbon Dioxide
If Sugar is heated gently it turns brown as some but not all of the Sugar is decomposed. The brown is from the Carbon dissolving into the sugar water solution. Carmel,and Molasses are forms of partially discomposed sugar.
If sugar is heated strongly it undergoes Oxidation a form of a composition reaction where the reactants combine with Oxygen. The Products are black Carbon, water and Carbon Dioxide.
When the marshmallow intended for the SomeMores, gets too close to the flame, the Marshmallow burst into flames itself. This is a decomposition reactions as well as an Oxidation.
Heating Sugar an organic covalent molecules causes the molecule to break apart. This is a decomposition reaction.
This question needs to have a fifth option: e) All of the above.
After discussion with a leading scientist on this site, we are of the opinion that the correct answer was given for the question as asked, but the question is not complete without All of the above.
Research into the topic here agrees with this premise:
More details are here:
If you plan to attempt this experiment, you will need an adult to learn with you as well as equipment to keep the experiment safe.
Some of the videos online are not safe - those spoons can get extremely hot (as a stove burner) and cannot be touched.
If you want to try another experiment in heating sugar, you might consider this site, where the result is not carbon black but something more delicious - help a parent to get involved: