Concept:Plural words refer to more than one. Some words have irregular plural forms that do not end in -s or -es.Explanation:"Alumni" is the plural form of "alumnus", meaning former male students or graduates. The other words are singular: "Oasis" becomes "oases", "Radius" becomes "radii", and "Broom" becomes "brooms". This is shown clearly in the table below.
Singular
Plural
Oasis
Oases
Radius
Radii
Broom
Brooms
Hence, only "Alumni" is already in its plural form.Answer:D. Alumni