"No, thank you!"
I'm a romanian and I naturally picked mersi, was marked as wrong :)) but it should have been accepted
Mersi is informal, mulțumesc is both formal and informal (used more in formal contexts). Since "thank you" is neither formal nor informal, both should be accepted.
Good to keep us on our feet, but why is this in the lesson about plural + definite article?
this program is seriously gonna quibble about a difference between "thanks" and "thank you"????