A Coulomb-excitation reorientation-effect measurement using the TIGRESS γ−ray spectrometer at the TRIUMF/ISAC II facility has permitted the determination of the 〈 2 1 + ‖ E 2 ˆ ‖ 2 1 + 〉 diagonal matrix element in 12C from particle−γ coincidence data and state-of-the-art no-core shell model calculations of the nuclear polarizability. The nuclear polarizability for the ground and first-excited (2 1 + ) states in 12C have been calculated using chiral NN N4LO500 and NN+3NF350 interactions, which show convergence and agreement with photo-absorption cross-section data. Predictions show a change in the nuclear polarizability with a substantial increase between the ground state and first excited 2 1 + state at 4.439 MeV. The polarizability of the 2 1 + state is introduced into the current and previous Coulomb-excitation reorientation-effect analyses of 12C. Spectroscopic quadrupole moments of Q S ( 2 1 + ) = + 0.053 ( 44 ) eb and Q S ( 2 1 + ) = + 0.08 ( 3 ) eb are determined, respectively, yielding a weighted average of Q S ( 2 1 + ) = + 0.071 ( 25 ) eb, in agreement with recent ab initio calculations. The present measurement confirms that the 2 1 + state of 12C is oblate and emphasizes the important role played by the nuclear polarizability in Coulomb-excitation studies of light nuclei.