Which of the following words is not an adjective?

Explanation

An adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified. Examples are "beautiful", "fast", good", "sweet" and "helpful". A tip for identifying adjectives is that you can put "to be" in front of the word: "To be beautiful", "she is beautiful", "the beautiful girl".