To be considered fresh water, water must contain salt in a concentration more than 30 parts per thousand less than 0.5 parts per million less than 0.5 parts per thousand more than 40 parts per million

Some sources claim that the limit is 1 part per thousand or even 3 parts per thousand. According to the most accepted definition of fresh water, the dissolved salts have to be in a proportion of less than 500 parts per million. This means that the proportion has to be less than 500/1000 parts per thousand, hence less that 0.5 parts per thousand.

The answer would be c