決めたの

did ... decide?

→ 決める
phrase Common #6,598
did ... decide? の gives the question a conversational, explanatory tone. The subject or object is understood from context.
JA: もう決めたの?
EN: Have you decided yet?

「誰がこれ決めたの?」

Who decided this?

— ムーンライズ・キングダム (2012)

さっき決めたの

I decided that earlier.

— 魔女の宅急便 (1989)

私決めたの

I've made up my mind.

— 魔女の宅急便 (1989)

それって、誰が決めたの?

Who decided that?

何するか決めたの?

Have you decided what to do?

私さキキくらいの時に—絵描きになろうって決めたの

You know, I decided I wanted to become an artist when I was about Kiki’s age.

— 魔女の宅急便 (1989)

誰がそんなこと決めたの?

Who decided that?

お昼どこで食べるか決めたの?

Have you decided where to have lunch?

何を決めたの?

What did you decide?

休みの日にどこ行くか決めたの?

Have you decided where you're going on your day off?