singular noun vs noun in (0 / a / the) singular form

[flyingwitch]

Hello.

Context: I am studying terms in linguistics.

Let's talk about the word "dog". Is the word:

1) a singular noun
2) a noun in singular form
3) a noun in a singular form
4) a noun in the singular form

? Thank you.

 

[entangledbank]

1) and 4), possibly also 2), and also a noun in the singular.