An intersystem crossing (ISC) is a non-radiative process that involves the transition between two electronic states with different spin multiplicity. That is, when an electron is excited in a molecule in a basal singlet state (either by absorption or radiation) into a state of greater energy, an excited singlet or triplet state can be obtained.
Therefore, ISC is understood as an a non radio active transition between states with different spin multiplicity.
Correct answer is C: a radiative transition between states with the same spin.
