A. The sick dog
It is a noun phrase because it is composed of a noun and a modifier.
The modifier can either come before or after the noun. In this case, it came before the noun.
If a modifier comes before the noun, it can either be an article, possessive nouns, possessive pronouns, adjectives, or participles. In this case, the modifier is an adjective.
Noun - dog
Modifier - the sick : it is an adjective that distinguishes the dog from other dogs.
