$\mathrm{CH}{}_3\mathrm{MgBr}$ reacts with propylene oxide (2-methyloxirane) at the less hindered ring carbon. After hydrolysis with $\mathrm{H}{}_3\mathrm{O}{}^{+}$, the product is

The nucleophile opens the epoxide in an SN2 fashion at the less substituted $\mathrm{CH}{}_2$ carbon. This gives $\mathrm{CH}{}_3\mathrm{CH}(\mathrm{OMgBr})\mathrm{CH}{}_2\mathrm{CH}{}_3$, and hydrolysis gives $\mathrm{CH}{}_3\mathrm{CH}(\mathrm{OH})\mathrm{CH}{}_2\mathrm{CH}{}_3$, butan-2-ol. Attack at the more substituted carbon would have given 2-methylpropan-1-ol.