We see that the compounds have the same atoms in their formula. However, they are not the same, because we cannot move the one on top of the other so that all atoms are the same. Specifically, the CH in the first case are not adjacent, while in the second case they are. They are constitutional isomers because they use the same chemical atoms in the same proportions, but their arrangement is different (they are not the reflection of each other too). There are examples of isomers in nature that have completely different properties. The correct answer is the 3rd choice.
