Answer:
It would be the first one.
Explanation:
First, locate the subordinating conjunction. In this sentence, it will be "because". "Because" is conjoining two sentences together that express a complete thought.
For example, if I break the sentence into two parts it would like this.
First sentence: Susan learned yoga.
Second sentence: She wanted to improve her flexibility.
If you add "because" you would make one whole sentence that would explain why Susan learned yoga.
Hope this helps!