Final answer:
The sentence with the subordinating conjunction is 'Susan learned yoga because she wanted to improve her flexibility,' where 'because' is the subordinating conjunction.
Step-by-step explanation:
The sentence that includes a subordinating conjunction is: "Susan learned yoga because she wanted to improve her flexibility." The word 'because' is a subordinating conjunction that introduces the reason why Susan learned yoga, making the clause that follows ('she wanted to improve her flexibility') a dependent or subordinate clause. This forms a complex sentence, which is a sentence that includes one independent clause and one or more dependent clauses.