"No, not this one!"
Translation:Nein, nicht dieser!
Why not "nicht diesem" ? Is it only using the feminine and plural forms of dative?
"Nein, nicht diesem" is correct, too.
Without context, i.e. without knowing what "this one" refers to, all genders and cases are possible and should be accepted as correct. Plural would normally not be possible, though, because the English sentence says "this one" and not "these" (although there might be words that are used in the singular in English and translate as plural in German).
I think it is because dies is the root word and needs an ending of either -e, -er, or -es. As Javi mentioned above, -em might work but the safer answer would be one of the first three.
I was marked wrong for, "Nein, nicht das einem." Is there some reason that, "Nein, nicht diesem," wouldn't translate better as, "No, not this," instead of, "No, not this one"?
"das einem" doesn't mean anything in german (as far as I know). to translate "this one" you need the pronoun "diese"