Directions: Pick the odd one out.

Cactus, walrus, alumnus, hippopotamus, syllabus

Correct. The four words - cactus, alumnus, hippopotamus, and syllabus– follow the same rule while forming the plurals (by changing –us of the singular into -i). Thus, the plural forms of these words are - cacti, alumni, hippopotami and syllabi. (Note: cactuses, hippopotamuses and syllabuses are also accepted.) The word ‘walrus’ is the odd one here since it forms the plural only by adding –es to the singular and is not related to the rule mentioned above.