Which energies are considered spin forbidden?

Spin forbidden transitions occur when the spin states of the initial and final configurations are different. This typically means that a transition from a singlet state (where all electrons are paired, with a total spin of zero) to a triplet state (where two electrons are unpaired, leading to a total spin of one) is considered spin forbidden.

In the context of energy levels, the lowest energy states are usually those that would correspond to the ground state configurations, which tend to be singlet states. Transitioning from these lowest-energy singlet states to a higher-energy state that is triplet is not allowed due to the rules of quantum mechanics regarding spin. Thus, the lowest energy transitions involving these singlet levels are classified as spin forbidden because they violate the spin selection rule.

Considering the other options, higher excited states, intermediate energies, and degenerate states may have their own transition characteristics, but they don't directly pertain to the spin selection rules in the same way as transitions from the lowest energy states, which are fundamental in understanding spin forbidden transitions.