Should be "jaki jest efekt"
This is the default answer. What was yours?
With "put the right words in the right order", it ended up being "Co to efekt". Is that not right?
It's also a valid translation, just with another meaning. In „Co to efekt?” you are asking somebody to define the word "effect" for you.
For the multiple-choice questions, you also have to recognize "skutek", which is not yet introduced, I believe.
Deleted from the best answers.