Water samples with BOD values of 4 ppm and 18 ppm, respectively, are:

We first recall the definition of BOD (Biological Oxygen Demand). It is the amount of dissolved oxygen, measured in parts per million (ppm), that microorganisms require to decompose the organic matter present in a given volume of water.

Environmental standards state that:

$$\text{If BOD} \lt 5\ \text{ppm, the water is regarded as \textit{clean}.}$$

$$\text{If BOD} \gt 5\ \text{ppm, the water is regarded as \textit{polluted}.}$$

Furthermore, when the BOD value rises well beyond $$5\ \text{ppm}$$, the pollution is often termed high. Typical textbooks label water with BOD values above roughly $$17\text{-}18\ \text{ppm}$$ as highly polluted.

Now we compare the given samples with these benchmarks.

For the first sample: $$\text{BOD} = 4\ \text{ppm}.$$

Since $$4\ \text{ppm} \lt 5\ \text{ppm},$$ we classify this water as clean.

For the second sample: $$\text{BOD} = 18\ \text{ppm}.$$

Here $$18\ \text{ppm} \gt 17\ \text{ppm},$$ so the water is definitely in the highly polluted category.

Putting the two results together, we have:

$$\bigl(4\ \text{ppm},\,18\ \text{ppm}\bigr) \;=\; (\text{Clean},\,\text{Highly polluted}).$$

Among the listed choices, this description corresponds to Option D: “Clean and Highly polluted.”

Hence, the correct answer is Option D.