"Your name is Marcus."
Translation:Nomen tibi est Marcus.
Your sentence reads like "Marcus is your name" making Marcus the main subject. Although it means the same thing the question is steering you towards making the name the subject, to make it read as "Your name is marcus" Technically your answer should be correct and it's just semantics. As others have said latin is very free in order like english
Marcus is the nominative form, used when Marcus is the subject or subject complement.
Nomen tibi est Marcus.
Marcus est senex.
Marce is the vocative form, used when you're calling out to Marcus, using his name to address him directly.
Quo vadis, Marce?
There was no need to post this three times. Once is enough.
There is nothing fishy about it. Different languages have different ways of saying things. Latin has both the genitive and the dative of possession, and that's what this lesson is teaching.
Here is a plain-English overview of what the cases are and how they work:
Latin cases, in English