Which of these is an example of an idiom?
a. i wasn't sure why he was angry. his mood came out of left field.
b. it was fun to watch my baby sister play in the field of grass.
c. the cat seemed angry. she hissed every time i came near.
d. my brother and i got in a huge fight. my best friend had to stop us.

Choice A. is an example of an idiom. Notice that it uses the words "his mood came out of left field." It's not to be taken literally. Idioms are expressions whose meaning is not predictable from the usual meanings.

Examples are:

"It's raining cats and dogs!"
- It's not literally raining cats and dogs like animals are falling from the sky. It means that the rain is heavy and it may refer to storm with winds.

"It costs me an arm and a leg."
- It doesn't really mean that you have to give up your body parts. It may refer to something so hard to get that you have to work really hard for it, causing your body to get tired and maybe it costs a lot that you need to give your body parts for it.

I hope you get it! :)

A. is the idiom because it has a word that is out of place and still describes the sentence