Here's why:
* Prepositions connect nouns or pronouns to other words in a sentence, showing their relationship. "Of" is a preposition.
* Prepositional phrases consist of a preposition and its object (the noun or pronoun that follows the preposition). "Proud" is the adjective that is modified by the prepositional phrase "of."
Example:
"She was proud of her accomplishments."
* "Proud" is the adjective.
* "of her accomplishments" is the prepositional phrase.
Therefore, "proud of" functions as a unit to modify the adjective "proud".