Phytoplanktons are photosynthesising microscopic organisms that inhabit the upper sun-lit layer of almost all oceans and freshwater bodies. Thus, growth of phytoplanktons in water bodies is not responsible for water pollution. 

Sedimentation method is not used to make water safe for drinking. It does not kill the harmful microbes present in water.

 The correct answer is (1).

Asthma is a common long-term inflammatory disease of the airways of the lungs.

The anomalous expansion of water is an abnormal property of water whereby it expands instead of contracting when the temperature goes from 4^oC to 0^oC and it becomes less dense. The density becomes less and less as it freezes because molecules of water normally form open crystal structures when in solid form.

Ice formed at the surface of water bodies protects the water beneath from the cold air above. Thus, the warmth of water below the layer of ice is preserved and not dissipated to the atmosphere.

It decreases oxygen level in water bodies.

The addition of a small quantity of potash alum helps the suspended particles to settle down quickly.

It decreases oxygen level in water bodies.

The correct answer is (4).