Mersi is wrong. There is a Romanian word for this. Multumesc.
No, "mersi" is correct. Moreover, it's better in this context, because "mersi" is less formal than "mulțumesc", similar to how "thanks" is less formal than "thank you".
Shouldn't "multumesc" be considered an acceptable answer anyway?
I don't see why not
Multumesc is surely also correct. It is not clear how formal or informal this situation is.
Multumesc is more common. Mersi is more informal, slang.
'Multumesc' and 'mersi' should both be accepted as they can both be used. Usually people say 'multumesc' though.