1. The only correct option is the first one. The second is wrong because of the mismatch between the singular 'a' and the plural 'fries'. The third and fourth options are wrong because 'French' always needs a capital 'F'. Whatever you think it means, the sentence "I want a French fry" means one single piece of fried potato. If you want a whole box of French fries, you have to ask for a box! 2. It isn't a sentence, because the two parts of it are unconnected. You could split it into two sentences, or you could keep it as one and join the two parts with "and" or "but", for example. Otherwise, you could get away with replacing the comma with a dash.

I hesitate to contradict Suki, whom I admire very much and who is the undisputed champion of excellent answers, but we’re talking about “french fries” — the American term (and one of my favorite foods). I’ll let the British speak for “chips.” In the States people often say “a fries” (short for an order of fries”), although it’s easy enough just to say “fries,” if you want to be grammatically safe. To specify the amount, we usually say “a small fries” or “a large fries” — not “few fries” or “many fries.” The size refers to the orders, not the individual pieces of potato. “French” should not be capitalized, since the fries are not really French at all — it’s just the name of the dish in American English. I’m not really a stickler for correct capitalization, but it appears the “Grammar Girl” agrees with me on this: https://www.quickanddirtytips.com/education/grammar/should-you-capitalize-the-french-in-french-fries?page=all

First part, (2), but without the 'a'... as it's in the plural form, the 'a' makes it incorrect. Also because 'French' is the adjectival form for France in this context, a proper noun, it needs to be capitalised. Alternatively, you could say 'a portion of French fries', which accomplishes the same thing, though the full noun phrase denotes a singular as you count the 'portions' and not the 'French fries'. Second part, as an imperative, yes, it's correct. Though it can be argued that it should be structured as two sentences, but owing to the presence of the comma, I reckon it's alright. You could replace the comma with 'and' however and it'll still be fine.