I see we are either being overly polite here or somehow we had to guess the app means the plural you with this question...
Yeah, I thought the answer was "Istutko sinä?". I was only able to determine they meant it in plural because one of the options was "Istutteko".
It accepted the singular "Istutko sinä?" as of 13/07/20
I had the same issue with this!
Is the pronoun necessary in phrases like this? May i just ask "istutteko"?
The pronouns 'minä', 'me', 'sinä', and 'te' may all be dropped. "Istutteko" was accepted just now.