The correct option is (d) Oxygen
Explanation: Bacteria like E. coli, Bacillus, Clostridium and Pseudomonas can methylate mercury in an area which is more enriched with sulphate. The habitats which have less sulphates or reduced form of sulphates, the methylation of mercury is not possible. However, oxygen doesn’t combine with mercury under normal temperature and hence, it cannot take part in the methylation of the mercury.