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What is the correct definition of "excited state"? a. The transfer of energy from a photon to an electron such that the electron ends up in a higher energy state. b. One of the discrete energies that a bound electron can have. c. A photon is a massless, chargeless particle with a discrete energy based upon its wavelength. d. The release of energy in the form of a photon as an electron transitions to a lower energy level. e. All energy states other than ground state until the ionisation threshold (E=0)