(R)-2-Methyloxirane is treated with aqueous NaCN (basic nucleophilic conditions). What is the product?

Cyanide attacks the less hindered $\mathrm{CH}{}_2$ carbon (SN2), so the stereocentre bearing O and $\mathrm{CH}{}_3$ is not touched and keeps its spatial arrangement. Its priorities are O $>$ $\mathrm{CH}{}_2$ (ring carbon, then $\mathrm{CH}{}_2\mathrm{CN}$) $>$ $\mathrm{CH}{}_3$ $>$ H in both compounds, so the descriptor stays R. Attack at the $\mathrm{CH}$ carbon would give 3-hydroxy-2-methylpropanenitrile, but this is not the pathway.