Countable nouns refer to things that can be counted, they have a singular and plural form, like apple/apples. Uncountable nouns refer to things that cannot be counted, because they are made up of so many minute particles, that makes it impossible to count. They do not have a plural form, example, rice, milk. From the options, A is correct because 'glass' can be counted, its plural form being 'glasses', and 'milk' cannot be counted, it can't be modified by a number but its quantity is measured by a quantifier like 'some milk', 'a little milk'. Option B is incorrect because 'glass' is not an uncountable noun, it is spoken in its plural form as 'two glasses', 'ten glasses', and 'milk' is not countable, you can't say 'two milks'. Option C is incorrect because milk is uncountable, but glass is not, it can be counted in number. Option D is incorrect because glass is countable, but milk is not, we can't use a number before milk, and it can't be made plural as well.