organic compound

organic compound may not have high no. of C atoms , eg. methene methane only have 1 C atom. ethene, ethane only have 2 C atoms.

organic compound may undergo incomplete combustion because some of them have many C atoms, so if it undergo complete combustion, there are not enoungh oxygen supply, when not enoungh oxgyen spply, it cannot form carbon dioxide from all C atom of the compound.
That means it there enough oxygen spply, it may undergo complete combustion

that's not a HKAL level's question(at my level 2006AL),
before answering your question, you should thinking about entropy.
entopy is defined as randomness of the system.
during combustion of hydrocarbon, bond breaken then form a shorter chain compound, continue heating cause chain effect, from high C no, to lowest C no.
at last, 2 state of oxidated carbon and C(s), are formed as the compond of lowest no. of carbon,
let us look, if the system a complete combustion, only CO2 will be formed, the randomness of the combustion was low becasue of the sys not enough "random"
in the case of lower C chain, probablity of forming CO2 still high. from incresing of C, the probablity tend to lower and lower........
reaction always prefer react on higher entropy's pathway, so ocurrancy of incomplete combustion increase if no. of C increase

When hydrocarbon combust, CO2 and H2O are formed. Incomplete combustion usually occurs in something like an internal combustion engine, where oxygen is limited. With organic compounds that have a high number of carbon atoms, there are some C atoms that cannot form CO2 due to a lack of oxygen, so CO and soot are created.