This sentence seems to have two correct answers:
"Mărul Anei este roșu" and "Mărul lui Anna este roșu".
Is there any difference between them? As far as I know, whenever a noun or a personal name ends in -a, it must change to -ei rather than follow the "lui [Nominative]" form.
Yes, that's the rule, the exception being boys' names that end in "a". However, some people do say things like "Mărul lui Anna este roșu" even if it's not correct.
This is incorrect! This is a popular expression, correct expression is: Mărul Anei este roșu .
If you select on of the 3 options The only way to respond correctly is to choose : Mărul lui Anna este roșu..
It would mean that Anna is a boy or you are in the street and speak local popular language.