Using an adjective as a noun in the plural form

Question

Which question is the more correct or the more colloquial? 1- "Why are they (those school subjects that you just mentioned) your FAVOURITE?" 2- or "Why are they (those school subjects that you just mentioned) your FAVOURITES?"

Answer 1

Correct: 'Why are they your favourites?'

Incorrect: Why are they your favourite? [A plural noun should be used with 'they'. favourite- singular noun, favourites- plural noun ]

Favourite (noun)- Why are those your favourites? (those school subjects that you just mentioned)

Favourite (adjective)- Why are those your favourite subjects?

-'Which are your favourite subjects?' -'My favourite subjects are English and History.

-'Which (subjects) are your favourites?' -'English and History are my favourites.'

Answer 2

'Favourite' is sometimes used alone as a noun, but is also used as an adjective alongside another noun:

• Which subject is your favourite?
• Which is your favourite subject?

When used alone, it is singular or plural depending on whether you are referring to a single thing, or many things:

• Which subject is your favourite?
• Which subects are your favourites?

But as part of a compound noun, it would be the other noun that is pluralised, if appropriate:

• Which is your favourite subject?
• Which are your favourite subjects?

In your example, it sounds like 3 subjects have been chosen as 'favourite' (most favoured). In this kind of example, it is also normal to use the singular favourite to ask about them collectively, so you could use either 'favourites' to refer to the subjects as individual items, or 'favourite' to refer to them collectively as the favourite subjects.