You can draw Hyper conjugating Structures to compare bond length. More the $\alpha$ Hydrogen of alkene, more are the Hyper conjugating structures, more is the delocalization of π-bond and more is the bond length (since double bond character decreases, bond length increases) Therefore answer should be (d). Hope it helps !

You can draw hyperconjugation structures to compare bond length. The more α-hydrogens the alkene has, the more hyperconjugation structures it has, and the higher is the degree of delocalization of the π-bond and therefore the longer is the bond. When double bond character decreases, bond length increases. Therefore the answer should be (d).