Note that butane has two possible isomers but that decane has 75 possible isomers. why does the number of possible isomers go up with an increasing number of carbon atoms?

Because the number of constitutional, conformational, and geometric isomers goes up with each carbon atom added. There are many more possible configurations and connectivities possible with decane, a 10 carbon chain, than with butane, a 4 carbon chain.

The answer should be vaporization 

The bergeron process.