Which conformation has the highest energy due to 0° separation of the two largest groups?

The totally eclipsed conformation has the highest energy because it presents a situation where the two largest substituents on adjacent carbon atoms are aligned directly with each other. In this conformation, there is significant steric hindrance due to the overlapping electron clouds of the larger groups, which increases the repulsive interactions between them.

In contrast, staggered conformations allow for the largest groups to be placed farther apart, minimizing steric interactions and lowering the overall energy of the molecule. The chair and boat conformations refer specifically to cyclohexane and involve different levels of steric strain and torsional strain. Among these conformations, neither the chair (which is the most stable) nor the boat (which has increased strain) would exhibit the extreme steric interaction seen in the totally eclipsed configuration. Therefore, it is the alignment of the larger groups in the totally eclipsed conformation that leads to the highest energy state due to the maximum steric hindrance.