The answer might be "us" because there might be an emphasis on the word.
Answer:
She couldn't have been more delighted if she wanted to be, and after she'd plunged into her newest sewing kit, she was perfectly content.
OR
He'd plunged into his newest gaming set so fast, he couldn't have been happier!
Explanation:
I assume they wanted you to describe the feelings, let me know if I'm right/wrong on this.
