Final answer:
Under acidic conditions, the epoxide becomes protonated to increase its electrophilicity, allowing the nucleophile to attack the more substituted carbon for greater stability.
Step-by-step explanation:
Under acidic conditions, the epoxide becomes protonated, and then the nucleophile attacks the more substituted carbon. In the acid-catalyzed mechanism, the acid protonates the less electrophilic substrate, making it a more electrophilic one by adding a proton to its oxygen atom.
This increases the positive character on the adjacent carbon, making it easier for a nucleophile to attack. Neutral nucleophiles require this activation step in an acidic medium since they're less reactive than their anionic forms. The nucleophilic attack generally occurs at the more substituted carbon due to the stability of the resulting carbocation intermediate.