The compound shown is a cis vicinal diol. It is treated with excess periodic acid. Which structure is the organic product (one molecule of product per molecule of reactant)?

The compound is cis-cyclopentane-1,2-diol. The cis diol forms a cyclic periodate ester and the C1-C2 bond breaks; since both carbons are in one ring, the ring opens to the dialdehyde pentanedial (glutaraldehyde). Aldehydes survive $\mathrm{HIO}{}_4$, so the diacid is not formed, and nothing remains to give a hydroxy-aldehyde or ketone.