d
\((d)\) It is possible to interconvert cis and trans alkenes, but the \(\pi\) -bond  must be broken first. This requires a considerable amount of energy - around \(260\, kJ\, mol^{-1}\) .One way to break the \(\pi\) -bond would be to promote an  electron from the \(\pi\) -orbital to the \(\pi^*\) -orbital. If this were to happen,  there would be one electron in the bonding \(\pi\) -orbital and one in the antibonding \(\pi^*\) -orbital and hence no overall bonding. Electromagnetic  radiation of the correct energy could promote the electron from \(HOMO\) to  \(LUMO\). The correct energy actually corresponds to light in the ultraviolet \((UV)\) region of the spectrum. Thus, shining \(UV\) light on an alkene would  promote an electron from its bonding \(\pi\) -molecular orbital to its antibonding  \(\pi^*\) -molecular orbital, thereby breaking the \(\pi\) -bond (but not the \(\sigma\) - bond)  and allowing rotation to occur.