"- Enjoy your meal, Anna! - Thanks!"

[ElRiXIV]

Mersi is wrong. There is a Romanian word for this. Multumesc.

[potestasity]

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".

[ValenCina]

Shouldn't "multumesc" be considered an acceptable answer anyway?

[Sammatazz]

I don't see why not

[MWGGordon]

Multumesc is surely also correct. It is not clear how formal or informal this situation is.

[Daniela622079]

Multumesc is more common. Mersi is more informal, slang.

[victoria_ania]

'Multumesc' and 'mersi' should both be accepted as they can both be used. Usually people say 'multumesc' though.