The difference between "think out" and "think up"

hello everybody!

I’ve met such a question like this:

What you have to do is to ___ a new idea that will enable us to make a lot of money.
a) think over
b) think out
c) think above
d) think up

The given answer is “D”, but some of us think “B” maybe work. Could you tell me which one is the best?

Many thanks!

It has to be “think up”. This expression means to invent a new idea by thinking.

“Think out” means to ponder over a problem or a situation that has to be resolved. It doesn’t involve creating anything, but analyzing something that already exists.

Thank you Jamie! You have enlightened me.