"You drink your water."
Translation:आप अपना पानी पीती हैं।
पीती has to agree with the subject, आप, not with पानी. Presumably, the "you" in this case is female.
"Aap apna paani piyo". That's what it should be. The answer given here is wrong. "Piti" is for a female. "Pita" is for male. "Aap" is gender neutral, as is the word "piyo".
Aap is gender neutral, but you know the gender of the person you are speaking to, so need to use the correct gender. And if you were addressing them as Aap, and it was a commend (which in this case it isn't, it's merely a statement) wouldn't you use the polite form pijiye, not piyo?