BOD value less than 5 ppm indicates a water sample to be pure because BOD is a measurement of organic waste present in water. If BOD is more than 5 ppm then it indicates more organic waste, leading dissolved oxygen to be less because of usage of oxygen in the degradation of organic waste by microbes.

On the other hand, (ii),(iii),(iv) are elucidating to the polluted water, anti to what has explained above, making them wrong.