Does increasing the size of an axial substituent always result in higher relative strain energies for the axial conformation

It is not necessary that the increase in the axial substituent size always enhances the strain energy for the axial conformation. In certain cases, the size of substituent increases due to which the bond length between the carbon and substituent also enhances.  

In this case, the relative strain energies for axial conformation does not increases. The example is iodocyclohexane.  

answer: c.

explanation:

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