Tamil Nadu is divided into seven agro-climatic zones, and each zone is identified by the soil types that dominate it.

Kancheepuram lies in the North Eastern Zone, where red sandy loam, clay loam and saline coastal alluvium are the main soils, so Kancheepuram pairs with red sandy loam (3).

Namakkal belongs to the North Western Zone, whose soils are non-calcareous red, non-calcareous brown and calcareous black, so Namakkal pairs with calcareous black (4).

Tiruchirappalli is part of the Cauvery Delta Zone, which is marked by red loamy soil and alluvium, so it pairs with red loamy (1).

Kanyakumari forms the High Rainfall Zone with saline coastal alluvium and deep red loam, so it pairs with saline coastal (2).

The sequence for (a), (b), (c), (d) is therefore 3, 4, 1, 2; every other code places at least two districts against soils that do not belong to their zone.

3 4 1 2 option is correct.