D. You can Waltz once, then you can do lots of kinds of waltzes. It's not A because we have a z at the end and that would make it sound the same. Not B because if you're aiming for plural words, we never put Zs. Can't be C because that would make it in possession. eg; That's the Waltz's song. Therefore, it must be D.
To me the key phrase is We found. That phrase makes it sound like it is a person speaking with another person, making We the pronoun for the un-named speaker.