Areaction of a hydrocarbon, in which carbon dioxide and water are produced, is classified as a
a. decomposition reaction.
c. double-displacement reaction.
b. combustion reaction.
d. synthesis reaction.

This is a combustion reaction. Any reaction in which a hydrocarbon decomposes into carbon dioxide and water is classified as combustion. The energy used for the decomposition usually comes from intense heat, which is why combustion of a hydrocarbon almost always is associated with a source of fire.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.