Number of possible open chain structural and configurational isomers with the molecular formula C₄H₈ is

 In chemistry, constitutional isomers specifically refers to compounds that have the same molecular formula but different structural formulae. 

They differ in the way that their atoms are joined together. This leads to different bonding and hence different properties, such as boiling point.

C₄H₈ can have five configurational isomers.

1- Cyclo-butane

2- Methyl cyclopropane

3- But-1-ene

4- But-2-ene

5- 2-Methylpropane

Hence, option( D) 5  is correct answer.