You're wrong there. There can be one word sentences in the English language. They can range from questions (Where? Why?) to commands (Go! Stop!) as well as statements (Nice.) I can correct you.
"The act of being so overcome with emotion that
1. The eyebrows come together and curve up at the inside.
2. The eyes become big and childlike
3. The lips and sometimes the chin shake.
Number three is what actually constitutes wibbling.
Sarah: Please come to the dance with me, Billy!
Billy: Uh...I dunno...
Sarah:...*wibbles*
Billy: Oh, Sarah! Of course I will!
Both: *luff*" http://www.urbandictionary.com/define.php?term=wibble
I don't necessarily agree with that, but even if I did, "Nice." would still not be a full sentence, even by that rule, since it's an adjective.
pls
Did you check the diagram thing with different sentences?
It said that "Excellent!" is a sentence, so "nice" which is just a lighter version of excellent would be good. And anyway, most words are acceptable as sentences overall, look it up.