"Is this rice?"
I am native speaker but this 个 sounds weird to me.
If I am asking about a single grain, I would say 这 粒/颗 是米吗？Other forms, other classifiers but 个 is never an option.
If I am using a demonstrative way without specifying the quantity (as in the English sentence) , I would say 这是米吗？, 这些是米吗？
这些米吗？is not good sentence. We need 是 in the middle.
这是米吗? and 这些是米吗？are basically the same. The different choices of demonstrative pronoun just refer to a single unit or a single category of things, versus multiple units/categories. The singular/plural concept here is only a contextual expression, and is not tied with the noun, as in English. If you see a single grain of rice, you should not use 这些; but if you see a pile of grains of the same type, you can use 这 (refering to a single type) or 这些 (referring to the number of grains). Conversely if you see several grains of different types, 这些 is more suitable.
这个是米吗？- it might looks correct, however I think that the natives won't use it. Because there's a word 密码 that sounds similar to 米吗. I think that they will prefer an option 这是不是米？ in this case. Well, if there are native speakers here - tell me if I guessed wrong. From my experience (I worked in China for couple years) chinese frequently use 是不是 construction for such cases, how I've heard