The large cluster of population in south asia is focused on the river basin, currently under threat from pollution and receding glaciers.

Ganges river basin      

Explanation:

Ganges river -

It is one of the popular river in the south asia , which originates from the mountain Himalayas , and flows entirely in the country India and then terminates at the Bay of Bengal .

The major threat to this river is the problem of pollution.

In Indian , the river is considered to be of huge spiritual importance , and hence , a lot of rituals are performed in the river , which in turns make the river dirty and polluted.

Hence , from the given information of the question,

The correct term is Ganges river basin.