Directions: Pick the odd one out.

Aquarium, decorum, medium, bacterium, memorandum

Correct. The four words - aquarium, medium, bacterium, and memorandum – follow the same rule while forming the plurals (by changing –um into –a). Thus, the correct plural forms of these words are - aquaria, media, bacteria and memoranda. (Though – aquariums, mediums and memorandums are also accepted.)The word ‘decorum’, though ending in –um, does not follow this rule of changing –um into -a. It forms the plural by adding –s.