Petroleum can be separated by distillation because the hydrocarbons in petroleum are
a) elements with identical boiling points
b) elements with different boiling points
c) compounds with identical boiling points
d) compounds with different boiling point

             Option-D (Compounds with different boiling point)

                    Compound is defined as a molecule which is made up of two or more different elements. All compounds are molecules but all molecules are not compounds, for example N₂, O₂, S₈, H₂, F₂ etc are molecules but they are not compounds.

                      Petroleum is a crude mixture of variety of hydrocarbons containing carbon and hydrogen as a primary elements along with other hetero elements like nitrogen, oxygen and sulfur etc. Therefore, these hydrocarbons can be termed as compounds because they are made up of atleast two essential elements like hydrogen and carbon.

                       Secondly, the process of distillation works on the basis of difference in boiling points of the compounds present in Petroleum. Therefore, compounds with lower boiling point will evaporate first and will separate first from the mixture and vice versa.