= Solution
<Protonation> converts OH into a water leaving group; ionization then gives the diarylmethyl cation. The labeled C-H/D bond is retained, so $k_H/k_D=1.18$ is a \b[normal alpha-secondary <kinetic isotope effect>], not a primary C-H cleavage effect.
At the labeled carbon, tetrahedral geometry becomes approximately trigonal planar. The associated change in bending vibrations and zero-point energies favors the H isotopologue in a substantially carbocation-like transition state. The two anisyl rings stabilize that cation and make this ionization pathway plausible.
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