"Я християнка, а мій друг - мусульманка." is given as a correct solution. But it would have to be "... моя подруга ..." wouldn't it?
Yes, you are right
Чому в цьому випадку "and" перекладається саме як "а"? І чому не може бути "і"?
"a" for distinction (my friend is Christian but by friend is Muslim) and "i" for conjunction (my friend and I are Muslims).
(And "в цьому випадку" in your question was a new phrase to me -- they should add it to Duolingo!)
What's wrong with "я християн і моя подруга мусульманка".
Nothing that I can see. The problem a year ago was that they gave the masculine pronoun with the feminine noun as a correct answer -- which I think has since been fixed.