Is Groundless an Adjective?

not based on reason or evidence synonym unfounded groundless allegations Our fears proved groundless.

Is stupendous a noun or adjective?

causing amazement; astounding; marvelous: stupendous news. amazingly large or great; immense: a stupendous mass of information.

Is retrospective noun or adjective?

Once you have "retrospective" behind you, you can also add its kin "retrospect" (which is used as a noun, an adjective, and a verb) and "retrospection" to your vocabulary, too. "Retrospective" can also be used as a noun, referring to an exhibition that "looks back" at artistic work created over a span of years.