The correct subordinating conjunction is "yet" and the sentence is completed as follows:
Mandy practiced karate every day, yet she failed to beat her final opponent.
The word "yet" is a subordinating conjunction that means "nevertheless" or "however", it is associated with negative statements in order to mention that an event did not take place.
In this sentence, it explains that even though Mandy had practiced she did not beat her opponent.
Answer:
It is either C or E, but definitely not A, B, or D
I'm not 100% on if it's C or E though, so I don't want to steer you wrong
