It's not a sentence, but a fragment. It needs a verb to complete it:

"Is that a flying saucer?".

"That" is a noun phrase, and so is "a flying saucer".


PaulM

Is 'That' a noun phrase because it refers to the flying saucer?

thank you for replying

No, it's a noun phrase because "that" is a pronoun (that's its part of speech), and pronouns are a sub-type of noun. We don't have a category 'pronoun phrase', so any phrase headed by a pronoun is called a noun phrase.


PaulM