Since the isomers are not given, I can't give an exact answer. But generally, the isomer that will have the smoothest combustion is the one that needs the least energy. Isomers are substances that have the same chemical formula, but differ in their structure. The more compact the structure is, the lesser energy for combustion in needs. For example, we have the isomers of pentane shown in the figure. The substance that can be easily combusted is 2,2-dimethylpropane, followed by 2-methylbutane and pentane.