 Solution

C, the neutral arachno ${\mathrm{B}}_6{\mathrm{H}}_{12}$, 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 $[{\mathrm{B}}_6{\mathrm{H}}_{11}{]}^{-}$. 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 $p{K}_a$ or a claim that all B–H hydrogens are equally acidic.