= Solution
\b[C, the neutral <arachno> $\mathrm{B_6H_{12}}$, is the most plausible <proton> donor among A, B and C.] Loss of an open-framework bridging <proton> produces the experimentally accessible, charge-delocalized conjugate base $[\mathrm{B_6H_{11}}]^-$. In contrast, deprotonating A or B would build a dianion and incurs additional electrostatic cost. The neutral, open hydrogen-rich cluster therefore has a more favourable route to a monoanionic conjugate base. This is a qualitative acidity prediction for the listed species, not a numerical solvent-independent $pK_a$ or a claim that all B–H hydrogens are equally acidic.
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