Translation:Is this food?
Very strangely, ...だか is not used to form a main clause question, although one can use ...だったか or ...だろうか, or simply put か after any other plain (/informal) verb or adjective. IMABI says だか can, however be used in subordinate clauses. (The description starts below example 28): https://www.imabi.net/theparticleka.htm
Even Samuel Martin in his massive Reference Grammar of Japanese (1975:923), doesn't seem to say much more than 'it is in doubt whether [main clause だか] ever normally occur[s in] complete sentences, for the ellipsis of da may be obligatory.'
So, here are a couple of plain (/informal, casual) style alternatives for polite questions ending in ですか:
- Make a question with intonation, dropping both だ and か. これはくだもの？
- Drop だ but keep か. This is a good option to express the A or B, fruit or plastic, type structure. これはくだものか、プラスチクか。But keep in mind that か in plain style sentences can sound a little rough, or masculine. かい instead of か would sound even rougher or low class.
- You can soften the tone of #2 by substituting な for だ and adding の (or also add か to give it a little edge again). これはくだものなの(か)、プラスチクなの(か)。
Some similar versions: そう？/ そうか / そうなの(か) [Is that so?] Remember そうかだろうか IS possible. 大丈夫 (だいじょうぶ)？大丈夫か / 大丈夫なの(か) [Are you okay?]
Similarly, you can make questions with plain verbs (past and non-past) other than だ / です, by adding か, の, or のか . But although you can make a question using か with the plain 'let's do / go ...' > 'Shall we do ...' form (=the hortative), I'm pretty certain you can't use の, or のか. Ex. しようか / しましょうか 行こうか / 行きましょうか。Finally, the reason there's no な in these versions for other verbs is because it replaces だ, which is not used or needed here.
Of course I understand what Hiba is saying about the literal content of the sentence, and it is up to the instructor to decide whether the purpose of a lesson is served better by focusing only on the narrow literal meaning, or on trying to understand how the sentence might be used pragmatically in the real world and alternative ways to say basically the same thing. Here I agree with Maeldryn about the likely intent of the sentence. Why would one ask 'Is this food?' You might want to know whether something is intended to be eaten or whether it is perhaps simply a decorative garnish. Or maybe you've just not ever eaten anything like it before where you're from. So you could ask this in many ways: Can you eat this? Are you supposed to eat this? ... Of course, as Hiba says, Japanese also has more literal ways to express these ideas, for example: これ食べられますか。
Actually, Duolingo is consistently inconsistent when it comes to small typos and marking you correct or incorrect. Sometimes you misplace one letter and it's marked correct and other times, sometimes the next question even, you make the same or a similar typo and Duolingo marks you wrong! Must be the day of the week ; )
Duolingo, at least now, marks a typo of 1 character in the whole sentence as correct, as long as the typo doesn't mean anything. Ex. "This is foos" is fine, "This if food" isn't because "if" is another thing and the user might actually be incorrect, "Tgis is foos" would also be incorrect, due to the multiple spelling errors.