With reference to ancient South India, Korkai, Poompuhar and Muchiri were well known as

1. A

   capital cities

   Capital cities. Madurai, Vanji, Uraiyur were the usual Sangam capitals.

2. B

   ports

   Ports. Korkai, Puhar (Poompuhar / Kaveripattinam) and Muziris (Muchiri) were the famous Tamil ports.

3. C

   centres of iron-and- steel making

   Iron-and-steel centres. Not the identity of this trio.

4. D

   shrines of Jain tirthankaras

   Jain tirthankara shrines. Unrelated.

Summary. Official key is (b) ports. Korkai (Pandya pearl port), Poompuhar / Puhar (Chola Kaveri mouth) and Muchiri / Muziris (Chera, Kerala) are the Sangam-era port trio.