I think it just means 'this' or 'it'. The 'is' is implied just like always. So it's literally, 'whose (is) this milk?'
I think it's because [milk - молоко] is a neuter noun. The interrogative pronouns ["whose?"] agree with the gender and number of the noun. Чиї is plural. Чиє is neuter.
In informal American English it works fine and means the same thing. It's better for people who take the course and their native language is not English if they do not accept too many informal answers.
For Carol&Farheen: maybe because "молоко" is a neutral noun, like in Russian...