Reason of water pollution is

Water pollution means harmful substances or excess waste entering water bodies — rivers, lakes, groundwater — and reducing their usability and ecological health. A question asking for the ‘reason of water pollution’ is really asking which of the listed factors genuinely add to that pollution load; when an exam item lists several such factors as separate options plus an ‘all of the above’ option, it is testing whether you recognise that more than one of them is a genuine contributor, not that only one is ‘the’ cause.

Checking each listed factor:

• Pesticides — pesticide and fertiliser use in farming washes into rivers, lakes and groundwater as runoff, and is a well-known source of water contamination.

• Human activity — industrial effluent, untreated sewage, and dumping of waste directly pollute water bodies.

• Population growth — more people means more sewage, more waste, and greater demand on agriculture and industry, which raises the pollution reaching water bodies.

Each of these — pesticide use, human and industrial activity, and population growth — is a genuine, separately-recognised contributor to water pollution; the question does not ask you to pick the single exclusive cause, so picking only one option would wrongly rule out the other valid causes. The correct choice is the one that includes all three together.