Final answer:
The bond between carbon and carbon (D) is not free to rotate due to electron resonance because it involves a double bond structure, where pi bonds prevent rotation without breaking the bond.
Step-by-step explanation:
The bond that is not free to rotate due to electron resonance is between carbon and carbon (D). This is because electron resonance involves the delocalization of electrons in a molecule that has a conjugated system, such as a double bond or a lone pair participating in conjugation.
In this case, the bond rotation is restricted due to the pi bonds that are part of the double bond system. The presence of a double bond suggests that electrons are shared in such a way that rotation around the bond would break the pi bond, which is not energetically favorable.
As an example, consider carbon dioxide (O=C=O) where the carbon atom is double bonded to two oxygen atoms. Here, the double bonds mean that electron pairs are shared between carbon and oxygen, making rotation around these bonds impossible without breaking the pi component of the double bonds.
For carbon-nitrogen bonds and carbon-oxygen bonds, if they are single bonds, they are typically able to rotate. It is only when double or triple bonds come into play that rotation is hindered due to the nature of the electron sharing and the geometric requirements it imposes.