Explanation:
Criterion is singular; criteria is plural. However, Merriam-Webster notes that usage doesn’t always comply. And we know what happens when that situation hits the tipping point — when’s the last time you heard someone mention an agendum?

Here’s the entry:
Pronunciation: \krī-ˈtir-ē-ən also krə-\
Function: noun
Inflected Form(s): plural -ria \-ē-ə\ also cri·te·ri·ons
Etymology: Greek kritērion, from krinein to judge, decide
Date: 1622

1 : a standard on which a judgment or decision may be based
2 : a characterizing mark or trait
synonyms: see standard
usage The plural criteria has been used as a singular for over half a century . Many of our examples, like the two foregoing, are taken from speech. But singular criteria is not uncommon in edited prose, and its use both in speech and writing seems to be increasing. Only time will tell whether it will reach the unquestioned acceptability of agenda.

Here’s a little quiz on criteria/criterion.

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Answer: a
Explanation: Criteria is plural, although the singular form, criterion, is much less often seen.