Final answer:
The underlined word 'long' in the sentence 'Such a long river!' is a Descriptive Adjective. It provides more information about the noun it is modifying ('river') by describing its length.
Step-by-step explanation:
In the sentence, 'Such a long river!', the adjective 'long' describes the noun 'river'. This type of adjective is known as a Descriptive Adjective as it provides more information about the qualities or features of the noun it modifies. Descriptive adjectives often describe size, color, shape, etc. In our example, the word 'long' provides information about the length of the 'river', so it tells us more about the river's attributes.
Learn more about Descriptive Adjective