Option 1: Capital cities - While these locations were significant in ancient South India, they were not considered capital cities. Although they had governance presence, they were not the ultimate centres for political power.

Option 2: Ports - This is the correct answer. Korkai, Poompuhar (also known as Kaveripoompattinam), and Muchiri (also known as Muziris) were ancient South Indian ports, which played crucial roles in trade, both locally and internationally with Romans, Greeks and others. Their strategic coastal locations enabled extensive maritime commerce.

Option 3: Centres of iron-and-steel making - This is incorrect. While iron and steel production was common in ancient India, these locations are not specifically associated with iron and steel production.

Option 4: Shrines of Jain Tirthankaras - This is incorrect. Although Jainism was widespread in various parts of India, these places are not specifically renowned as Jain shrines nor associated prominently with Jain Tirthankaras. Their significance lies in their status as important ports.