In Mizoram, the settlement pattern is mostly of ‘linear type’ along the ridges, because

Answer: (a) the valleys are cooler than the ridges

The key says valleys are cooler — cold air drains into the valleys at night, and they are damp.

The idea

On calm, clear nights, slopes lose heat and the cooled, dense air can drain into valleys. Valley bottoms can then be colder than slopes or ridges above them. This is a local temperature inversion, where temperature rises with height through a shallow layer. It is a conditional night-time pattern, not a rule for every hour.

Clear, calm night → Ground loses heat by radiation → Air near the slope cools → Dense air drains downhill → Cold air collects in valleys → Ridge sites can remain warmer than valley bottoms

Why the others are wrong

(b) Accessibility is a factor but not the key’s answer.

(c) Ridges are warmer than valleys at night (temperature inversion).

(d) Dense forest is not the main reason.

The trap

Temperature inversion: on hill slopes, cold air sinks into valleys.

Also know

Linear settlements follow narrow features such as ridges, roads, canals or coastlines. Compact settlements cluster buildings; dispersed settlements spread homes across farmland or slopes. Hilltop settlement can also reflect drainage, defence and access, so temperature alone does not explain every ridge village.