For an atom of hydrogen without the effects of an external magnetic field, an electron transition from the 3P subshell to the 2P subshell is called a forbidden transition. Why is this so? a) An atom that changes energy through transitioning must either absorb or emit a photon. b) Electrons in hydrogen are forbidden from transitioning in that way due to peculiarities unique to solutions of Schrödinger's equation for hydrogen. c) Since the electron at low energies must pass through the nucleus, nuclear effects prevent certain transitions to orbits through the nucleus. d) The normal Zeeman effect prohibits certain transitions from occurring.