You are at a video rental store with your children. You are getting several DVDs to watch over the weekend. You are going to let your children choose one DVD to watch. You say:
Pick out which one you want.
To "pick out" something means to choose what you want. You use this phrase when talking about choosing from among a lot of different options. It's often used when shopping:
I picked out a nice dress for you to wear to Aiko's graduation.
You don't use the phrase "pick something out" when you have only 2 or 3 options.
It's also more casual to say "pick out" than to use the word "choose".
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