The correct answer is A. Benzene.
Benzene is a colorless, highly flammable liquid with a sweet odor. It is a member of the aromatic hydrocarbons, and is a major component of gasoline. Benzene is a simple molecule, consisting of six carbon atoms arranged in a ring with one hydrogen atom attached to each carbon atom.
When benzene is burned, it reacts with oxygen in the air to form carbon dioxide and water. The reaction is very exothermic, and produces a lot of heat. However, the reaction does not produce any soot, which is a black, powdery substance that is often seen when other hydrocarbons are burned.
The other options, hexane, naphthalene, and anthracene, are all alkanes. Alkanes are hydrocarbons that contain only carbon and hydrogen atoms, and are arranged in a straight chain. When alkanes are burned, they react with oxygen in the air to form carbon dioxide and water. The reaction is also very exothermic, and produces a lot of heat. However, the reaction does produce soot, which is why alkanes are often used as fuels in engines.
In conclusion, benzene is the only option that will not give a sooty flame when burned.