Explanation:

- In the sentence, the word "dull" describes the grey clouds.

- The word is modifying "grey clouds" by providing more information about their appearance.

- This descriptive function indicates "dull" is an adjective because adjectives describe or modify nouns.

Options:

1. Noun: This refers to a person, place, or thing. "Dull" does not fit this category.

2. Adverb: Adverbs modify verbs, adjectives, or other adverbs. "Dull" is not performing this function here.

3. Verb: A verb represents an action or state. "Dull" is not acting as a verb in the sentence.

4. Adjective: ? This option is correct as "dull" is describing the clouds.