A. Criterion is the singular version, and it is irregular because it is a Greek word in which the –on is changed to –a in its plural form.
Explanation:
"Criteria" is the irregular plural form for the word "criterion", which means a principle, benchmark or standard by which something is judged. "Criterion" is a Greek word, which is why it has this irregular plural. Another Greek word that has the same -on ending and uses the same -a plural is "phenomenon"/"phenomena". With that in mind, we can safely choose letter A as the correct option.
