Which group of words is an example of an idiom? wipe your face

when pigs fly

run fast

the first day of school

An idiom is a phrase or expression that typically presents a figurative, non-literal meaning attached to the phrase

So the answer should be
2.) when pigs fly

It describes something that is very unlikely or impossible to happen.

A. an is indefinite article

  i think you forgot to put the options but if your reffering to this question then your answer will be "b"  a. add "to" to the base form to create an adjective.b. when the subject of a sentence is singular, add "–s" to the base form.c. add "–ed" to irregular base forms to form the past participle.d. do not use a base form with plural subjects