Question

milk cost 50 rupees _ litre fill article​

Answer 1

The blank can be filled with the word 'a'

So the correct sentence will be

Milk cost 50 rupees a litre.

There are three articles i.e. a, an and the. The article is the word that is used with noun to show it's grammatical definiteness.

A and an are used when the noun us singular. These are used on the basis of pronunciation sound.

Here cost of one litre of milk is considered so we will use 'a' litre as litre has consonant sound.