The alpine vegetation in the Western Himalayas is found only up to a height of 3000 metres while in the Eastern Himalayas it is found up to a height of 4000 metres. The reason for this variation in the same mountain range is that:

Answer: (b) Eastern Himalayas are nearer to the equator and seacoast than the Western Himalayas

Lower latitude makes the east warmer at the same altitude, so every vegetation belt, including the alpine, sits higher.

The idea

Mountain vegetation belts mainly follow temperature, which generally falls with altitude. A lower-latitude mountain can remain warmer at a given height, allowing a belt to extend higher. Moisture and exposure also matter, so a belt’s boundary is not fixed at one height everywhere.

Lower latitude → Warmer conditions at comparable altitude → Temperature falls with increasing height → Cold threshold is reached farther upslope → Vegetation belts extend higher

Why the others are wrong

(a) Height of the peaks does not decide where alpine vegetation stops.

(c) More rain changes forest density, not the temperature limit of a belt.

(d) Rock fertility has nothing to do with altitude zones.

The trap

Rain is the famous difference between the east and west Himalaya, but belts follow temperature.

Also know

Alpine vegetation grows above the tree line and below areas where permanent snow prevents normal plant growth. The tree line and snowline are different boundaries. Aspect matters: sunny slopes and shaded slopes can have different local limits even in the same valley.