Intersystem crossing is:

a) a nonradiative transition between states with the same spin.
b) a radiative transition between states with different spin.
c) a radiative transition between states with the same spin.
d) a nonradiative transition between the vibrational levels of the same electronic state.
e) a nonradiative transition between states with different spin.

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.