Translation:What do you want to drink? Coffee or glögi?
It's all explained well here: https://uusikielemme.fi/finnish-vocabulary/interesting-words/the-difference-between-mika-and-mita-question-words Generally, you use mikä when the subject is well defined and concrete. You use mitä when it's more abstract or uncountable, that's also when you use the partitive form. As I learned, you always use mitä with another verb than olla. E.g. Mitä sinä teet. But, Mikä tämä on?
Same question. I am not a fan of the drink, but I always thought glögi & mulled wine are the same thing. But since it does not accept it as correct then is glögi a specific mulled wine? Or should it be accepted as correct (even if it's not the specific word learn in this lesson... which they always seem to prefer).