Final answer:
The underlined word 'red' in the given sentence is an adjective that describes the color of the car.
Step-by-step explanation:
In the sentence "The bright, red car was parked by the mailbox,", the underlined part is an adjective. Adjectives modify nouns by giving more information about their qualities, quantity, or state. Here, 'red' describes the color of the car, therefore functioning as an adjective that modifies the noun 'car'. It is not a verb, as it does not show action; it is not an adverb, as it does not modify a verb, adjective, or other adverb; and it is not a preposition, as it does not show a relationship between nouns or pronouns in the sentence.