The first step of the acid-catalysed hydration of propene is addition of $\mathrm{H}{}^{+}$ to the double bond to give a carbocation. The proton goes to the terminal carbon because

1. Athe step is endothermic and its transition state resembles the carbocation, so the more stable secondary cation is reached through the lower transition state
2. Bthe primary cation is more stable than the secondary one
3. Cthe transition state resembles propene itself, so cation stability is irrelevant
4. Da secondary cation has fewer hyperconjugating C-H bonds