There's nothing in the English sentence that suggests the plural should be used for You :/
"Kuuletko?" should be accepted too...
Kuuletko is also correct
No need to write about it again, just report it. (Check to see you didn't make some other error before reporting). The course contributors will eventually find it and add it, and you'll get a thank you email. =D
Reporting this, sept 18. I keep writing kuuletko