The sentence requires a phrase that introduces a contrasting idea, similar to "However" in Karen's original statement.

The sentence requires a phrase that introduces a contrasting idea, similar to "However" in Karen's original statement.

• "but in her opinion": This phrase introduces a contrasting viewpoint. Karen agrees the concert was amazing, but she has a different opinion about their earlier music. This fits.
• "since she thinks": This phrase introduces a reason. It implies that her thinking their earlier music is best is the reason the concert was amazing, which is incorrect.
• "because she is sure that": This phrase also introduces a reason. It implies that her certainty about their earlier music being best is the reason the concert was amazing, which is incorrect.
• "although she feels that": This phrase introduces a concession or a contrasting idea. It means "despite the fact that." Karen agrees the concert was amazing, although she feels their earlier music is still the best. This fits.

The two correct answers are:

• but in her opinion
• although she feels that