Which of the following word is plural?

To identify which of the provided words is plural, we can analyze the singular and plural forms for each option. In English, many nouns derived from Latin or Greek have irregular plural forms.

Analysis of the Options
Oasis is a singular noun. Its plural form is "oases," where the 'is' suffix changes to 'es'.

Radius is a singular noun. Its plural form is "radii," where the 'us' suffix is replaced by 'i'.

Stadium is a singular noun. Its standard plural form is "stadiums," although "stadia" is also sometimes used in technical contexts.

Alumni is the plural form of the singular noun "alumnus." Since "alumnus" refers to one male graduate, "alumni" refers to a group of graduates.