Most of the power plants dispose of their waste water, used

Report says that uranium makes up for an ever increasing percentage of water pollutants (i.e. the % of uranium in the total amount of pollutants is increasing). Note here that it doesn't say that % of uranium in water samples is increasing.
e.g. if uranium made up 2% of the total pollutants before, it now makes up 3% of the total pollutants.
What is happening to the % of total pollutants in the water is not known.

Conclusion: Efforts to decrease the amount of uranium are not working.

We want to weaken the conclusion i.e. we want to show that the efforts could be working.

Option (C) says that the proportion of other major water pollutants is decreasing. This could explain the increase in the proportion of uranium in the pollutants.

Think of values to understand this:

Say, % of total pollutants in the water sample is 5% i.e. in 100 lts of water, there's 5 lts of pollutants.
The 5 lt is made up of uranium, arsenic etc. Say, uranium is 1 lt and the others are 4 lts i.e. uranium is 20% of the total pollutants.
Now what if the amount of other pollutants go down from 4lts to 1 lt due to govt's efforts and amount of uranium goes down from 1 lt to 0.5 lt. The proportion of uranium has gone up to 33% of the total pollutants.
Even though proportion of uranium has increased, the efforts are working since the amount of uranium pollutant went down. Hence we have weakened the conclusion.