Sewage treatment in an oxidation pond is accomplished primarily by :

ANS:A - alga-bacterial symbiosis

Bacteria decomposes organic matter and the by-products of that decomposition such as Co2, ammonia, nitrates are used by algae to accomplish photosynthesis and thus produce oxygen which in turn is used by bacteria to decompose more organic waste.

So, option A is correct.

Although as per latest studies, even fungi are involved in the cleaning process.