Concept:Hybridisation of a carbon atom depends on the number of sigma bonds and pi bonds it forms.Explanation:The given compound is 1CH2=2CH−3CH2−4CH2−5C≡6CHCarbon C2 forms a double bond, so it has one π bond and three σ bonds. Hence, C2 is sp2 hybridised.Carbon C3 forms only single bonds with C2, C4, and two hydrogen atoms. Thus, C3 has four σ bonds and is sp3 hybridised.Therefore, the C2−C3 bond is formed between an sp2 carbon and an sp3 carbon.Answer:The C2−C3 bond is of type sp2−sp3, which is Option D.