What type of molecule is propane??

It's an Alkanes molecule. Alkanes saturated hydrocarbons which means they are made from hydrogen and carbon. Saturated means the have two additional hydrogen atoms which prevents them from being strung together like Alkenes. (which are bonded into long chains to make polyethylene ) Their size depends on the number of carbon atoms in the molecule. CnH(2n+2) Propane has 3 carbon atoms. C3H8