Answer:
The correct answer is "however".
Explanation:
In the example provided, we need a conjunction that expresses contrast.
Mandy practiced karate every day; however, she failed to beat her final opponent.
In order to link the two independant clauses into a compound sentence, the adverb "however" or "nevertheless"can be used and are correct.
If we want to change an independant clause by a subordinating one, the sentence would be the following (also expressing contrast):
Even though Mandy practiced karate every day, she failed to beat her final opponent.
